IgG against Desmoglein 3 (& 1) -> intraepidermal acantholysis, flaccid bullae, oral mucosal erosions, positive Nikolsky sign, 'tombstone' basal cells, net-like intercellular IF.
IgG against hemidesmosomes (BP180/BP230) -> subepidermal tense bullae with eosinophils, spares oral mucosa, negative Nikolsky sign, linear IgG/C3 at dermoepidermal junction.
Pathognomonic for Celiac disease: pruritic grouped vesicles on elbows/knees/buttocks; granular IgA deposits at tips of dermal papillae. Rx: Dapsone (acute) + Gluten-free diet.
Uroporphyrinogen decarboxylase deficiency (Hep C, alcohol, iron overload): painless blisters/scarring on sun-exposed dorsal hands, facial hypertrichosis, tea-colored urine. Rx: Phlebotomy.
<10% BSA epidermal detachment = SJS; 10-30% = SJS/TEN overlap; >30% = TEN. Positive Nikolsky sign + >=2 mucosal sites. Triggered by allopurinol, lamotrigine, carbamazepine, sulfa.
Well-demarcated salmon plaques with silvery scale on extensor elbows/knees/scalp, Auspitz sign (pinpoint bleeding), Koebner phenomenon, nail pitting, Munro microabscesses.
Pruritic, Purple, Polygonal, Planar Papules and Plaques on wrists/ankles with lacy white Wickham striae (oral/cutaneous) and sawtooth dermoepidermal lymphocyte infiltrate. Linked to Hep C.
Severe nodulocystic acne: 13-cis-retinoic acid requires iPLEDGE (2 negative pregnancy tests + 2 contraception methods); causes teratogenicity, hypertriglyceridemia, dry mucosae. Avoid tetracyclines (pseudotumor cerebri!).
Autoimmune Blistering Dermatoses Master Comparison
Autoimmune blistering dermatoses (immunobullous diseases) are differentiated by the anatomical depth of the epidermal cleavage plane: intraepidermal acantholysis above the basal layer produces thin-walled, flaccid blisters that rupture easily (Pemphigus Vulgaris), whereas subepidermal cleavage at the dermal-epidermal junction produces thick-walled, tense bullae that remain intact (Bullous Pemphigoid and Dermatitis Herpetiformis). Bedside Nikolsky testing and Direct Immunofluorescence (DIF) microscopy establish the diagnosis.
| Disorder | Target Antigen & Level of Blister | Nikolsky Sign & Mucosal Involvement | Direct Immunofluorescence (DIF) Pattern | First-Line Therapy |
|---|---|---|---|---|
| Pemphigus Vulgaris | Desmosomes (anti-desmoglein 1 and 3); Intraepidermal acantholysis with 'tombstoning' of basal keratinocytes | Nikolsky positive (lateral pressure induces new blister/skin sloughing); oral mucosal involvement almost always present | Intercellular IgG and C3 deposition throughout epidermis ('net-like' / 'fishnet' pattern) | Systemic high-dose corticosteroids (Prednisone) + Rituximab (anti-CD20) |
| Bullous Pemphigoid | Hemidesmosomes (BP180 and BP230); Subepidermal blister with eosinophil-rich infiltrate | Nikolsky negative; tense bullae that resist rupture; mucosal involvement rare (< 20%) | Linear IgG and C3 deposition along the epidermal basement membrane zone | High-potency topical corticosteroids (Clobetasol) or oral Prednisone |
| Dermatitis Herpetiformis | Epidermal transglutaminase-3; associated with Celiac disease (HLA-DQ2/DQ8) | Extremely pruritic microvesicles grouped on extensor surfaces (elbows, knees, buttocks) | Granular IgA deposition in dermal papillae | Strict gluten-free diet + oral Dapsone (check G6PD level prior to initiation) |
Pemphigus Vulgaris (Anti-Desmoglein 3/1, Flaccid Nikolsky+, Oral Mucosa & Tombstone Net-Like IgG) vs. Bullous Pemphigoid (Anti-BP180 Hemidesmosome, Tense Nikolsky- & Linear IgG)
The single highest-yield dermatology board comparison contrasts Pemphigus Vulgaris (PV) and Bullous Pemphigoid (BP) across five features: (1) Autoantigen & Structural Target: in Pemphigus Vulgaris, Type II IgG autoantibodies target Desmoglein-3 (and Desmoglein-1)—cadherins within Desmosomes that hold epidermal keratinocytes together across the stratum spinosum (acantholysis); in Bullous Pemphigoid, IgG autoantibodies target BP180 (Type XVII collagen) and BP230 within Hemidesmosomes that anchor basal keratinocytes to the basement membrane lamina lucida. (2) Blister Morphology & Nikolsky Sign: because the roof of a Pemphigus Vulgaris blister consists only of thin superficial epidermis, PV presents in adults aged 40–60 with FLACCID, fragile intraepidermal bullae that rupture immediately into painful, raw, crusted erosions and a POSITIVE Nikolsky Sign (gentle lateral finger pressure on unblistered perilesional skin shears the epidermis off!). Conversely, because the roof of a Bullous Pemphigoid blister consists of the entire full-thickness epidermis, BP presents in elderly patients (>65–75 years; more common than PV) with TENSE, intact subepidermal bullae on an erythematous/urticarial base (flexor surfaces, groin, axillae) and a NEGATIVE Nikolsky Sign. (3) Oral Mucosal Involvement: Pemphigus Vulgaris ALMOST ALWAYS involves the oral mucosa (painful oral ulcers often precede cutaneous blisters by months!), whereas Bullous Pemphigoid SPARES the oral mucosa in ~80%–90% of cases. (4) Histopathology & Direct Immunofluorescence (DIF): PV biopsy shows suprabasal acantholysis leaving a single row of basal keratinocytes attached to the basement membrane like a 'Row of Tombstones', and DIF shows intercellular 'Net-Like / Chicken-Wire / Fishnet' IgG and C3 fluorescence throughout the epidermis; BP biopsy shows subepidermal eosinophil-rich cleavage, and DIF shows a continuous LINEAR band of IgG and C3 along the Dermal-Epidermal Junction. (5) Treatment: PV carries higher mortality (fluid/electrolyte loss and secondary infection) and requires systemic Corticosteroids + Rituximab (first-line steroid-sparing anti-CD20!) or Azathioprine/Mycophenolate; localized BP can often be managed with High-Potency Topical Clobetasol or systemic prednisone/doxycycline.
Dermatitis Herpetiformis (Celiac IgA Dermal Papillae), Porphyria Cutanea Tarda (UROD / HCV / Tea-Colored Urine) & Erythema Multiforme
Three additional vesiculobullous conditions are tested alongside PV and BP: (1) Dermatitis Herpetiformis (DH) is the cutaneous manifestation of Celiac Disease (100% have subclinical or symptomatic gluten-sensitive enteropathy, even if GI symptoms are absent!), presenting with intensely pruritic grouped vesicles, papules, and excoriations symmetrically distributed over the extensor elbows, knees, shoulders, and buttocks/sacrum; skin biopsy shows subepidermal neutrophil microabscesses at the dermal papillary tips, and DIF shows pathognomonic granular IgA deposits at the tips of the dermal papillae (treat with a lifelong Gluten-Free Diet + Dapsone for rapid acute itch/blister relief; check G6PD before dapsone!). (2) Porphyria Cutanea Tarda (PCT) is the most common porphyria, caused by congenital or acquired deficiency of hepatic Uroporphyrinogen Decarboxylase (UROD)—strongly triggered by Chronic Hepatitis C Virus (HCV) infection, Alcohol use, Estrogen/OCP therapy, Smoking, and Hereditary Hemochromatosis (HFE iron overload). Accumulating photosensitizing porphyrins cause NON-healing, painless blisters, skin fragility, milia, and hypertrichosis (facial hair growth) strictly on SUN-EXPOSED skin (especially the dorsal hands and forearms), accompanied by reddish-brown ('tea-colored' or pink-fluorescent under Wood lamp) urine with elevated 24-hour urine uroporphyrin; treat with Phlebotomy (to reduce hepatic iron), low-dose Hydroxychloroquine, sun avoidance, and curing HCV! (3) Erythema Multiforme (EM) presents with acute symmetric 'Targetoid' lesions (three concentric zones: dusky central necrosis/vesicle, pale edematous middle ring, and erythematous outer halo) on the palms, soles, and extremities; unlike SJS/TEN (which is drug-induced), Erythema Multiforme is triggered in >90% of cases by infection—#1 Herpes Simplex Virus (HSV) and #2 Mycoplasma pneumoniae!
Severe Cutaneous Adverse Reactions (SCAR): SJS vs. TEN vs. DRESS
Severe Cutaneous Adverse Reactions (SCARs)—encompassing Stevens-Johnson Syndrome (SJS), Toxic Epidermal Necrolysis (TEN), Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS), and Acute Generalized Exanthematous Pustulosis (AGEP)—are delayed Type IV T-cell–mediated hypersensitivity emergencies. Rapidly calculating the percentage of detached Body Surface Area (BSA <10% vs. 10%–30% vs. >30%), checking for mucosal involvement, and assessing peripheral eosinophilia/visceral organ hepatitis differentiates these syndromes from simple morbilliform drug eruptions and Staphylococcal Scalded Skin Syndrome.
| Syndrome | High-Risk Triggering Medications | Pathophysiology & BSA Sloughing | Systemic Involvements & Key Markers |
|---|---|---|---|
| Stevens-Johnson Syndrome (SJS) | Allopurinol, Anticonvulsants (Carbamazepine, Lamotrigine, Phenytoin), Sulfonamides, Nevirapine, NSAIDs | Fas/FasL and perforin/granzyme cytotoxic T-cell death; < 10% BSA detachment; ≥ 2 mucosal surfaces involved | Severe oral/conjunctival erosions, hemorrhagic crusting of lips; Nikolsky positive; SCORTEN mortality score |
| Toxic Epidermal Necrolysis (TEN) | Same triggers as SJS; HLA-B*1502 in Asians taking Carbamazepine | Confluent sheet-like detachment > 30% BSA with full-thickness epidermal necrosis | Emergency transfer to ICU / burn unit; IVIG, Cyclosporine, or Etanercept; strict fluid and thermal balance |
| DRESS Syndrome | Anticonvulsants (Phenytoin, Carbamazepine), Allopurinol, Sulfonamides, Vancomycin; 2–8 weeks latency | Drug Reaction with Eosinophilia and Systemic Symptoms; associated with HHV-6 reactivation | Fever, diffuse morbilliform rash, facial edema, diffuse lymphadenopathy, marked eosinophilia (> 700), acute interstitial nephritis or hepatitis |
SJS (<10% BSA) vs. TEN (>30% BSA): Culprit Drugs, CD8+ Keratinocyte Apoptosis, Burn-Unit Care & SSSS (Exfoliative Toxin) Contrast
Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN) represent a single disease spectrum of CD8+ cytotoxic T-cell and NK-cell hypersensitivity (mediated by granulysin, perforin/granzyme B, and Fas-FasL) that induces full-thickness epidermal keratinocyte necrosis and subepidermal sloughing 1 to 4 weeks after starting a high-risk medication: Allopurinol, Anticonvulsants (Lamotrigine, Carbamazepine, Phenytoin, Phenobarbital), Sulfonamide antibiotics (TMP-SMX), NSAIDs (piroxicam), or Nevirapine (or Mycoplasma pneumoniae in children). Both begin with a 1–3 day influenza-like prodrome (fever, malaise, photophobia, burning eyes, odynophagia) followed by painful coalescing dusky red macules, POSITIVE Nikolsky sign, and severe hemorrhagic erosions of ≥2 MUCOSAL SURFACES (oral/labial crusts, purulent conjunctivitis/corneal ulceration, and urogenital mucosa). Classification is determined strictly by the percentage of Body Surface Area (BSA) with epidermal detachment: SJS = <10% BSA detachment (mortality ~5%–10%); SJS/TEN Overlap = 10% to 30% BSA detachment; and TEN = >30% BSA detachment (mortality ~30%–50%, predicted by the SCORTEN score). Immediate management requires stopping the offending drug, transferring the patient to a Burn ICU for fluid/electrolyte resuscitation, wound care, and ophthalmology consultation (to prevent symblepharon/blindness), plus cyclosporine/etanercept/IVIG per center protocol. CRITICAL BOARD CONTRAST: Distinguish SJS/TEN from Staphylococcal Scalded Skin Syndrome (SSSS), which occurs in neonates and young children (<5 years) due to phage-group II Staphylococcus aureus Exfoliative Toxins A and B (proteases that cleave Desmoglein-1 ONLY in the superficial stratum granulosum!): SSSS presents with fever, diffuse scarlatiniform tenderness, perioral crusting, and a POSITIVE Nikolsky sign, BUT SSSS STRICTLY SPARES THE MUCOSAL SURFACES (no oral/ocular erosions because mucosa expresses Desmoglein-3!) and heals without scarring using IV Nafcillin/Oxacillin or Vancomycin!
DRESS Syndrome (2–8 Week Latency, Eosinophilia, Facial Edema & Hepatic/Renal Injury) vs. Morbilliform Drug Eruption
When a patient develops a diffuse drug rash after starting an anticonvulsant, allopurinol, vancomycin, minocycline, or sulfonamide, boards test how to distinguish a benign Simple Exanthematous (Morbilliform) Drug Eruption from life-threatening DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms): (1) Simple Morbilliform Drug Eruption (90% of all drug rashes; Type IV Th1/CD4+ reaction to beta-lactams/cephalosporins/sulfonamides) appears 4 to 14 days after starting the drug as a symmetric maculopapular ('measles-like') truncal/extremity rash with pruritus, NO fever >38.5°C, NO facial edema, NO mucosal blisters, NO lymphadenopathy, normal eosinophils, and COMPLETELY NORMAL liver transaminases and renal function (resolves within 1–2 weeks of stopping the drug with topical steroids and antihistamines). (2) DRESS Syndrome is characterized by a uniquely prolonged 2-to-8-week latency period after starting the offending medication (classically Allopurinol, Carbamazepine, Lamotrigine, Phenytoin, Vancomycin, Dapsone, or Sulfonamides, often accompanied by HHV-6 reactivation!) and presents with the diagnostic tetrad of: (a) High Fever (>38.5°C) and Diffuse Morbilliform Eruption with prominent FACIAL AND PERIORBITAL EDEMA; (b) Generalized Lymphadenopathy; (c) Hematologic Abnormalities: Marked Peripheral EOSINOPHILIA (>700–1,500/μL) and Atypical Lymphocytosis; and (d) Internal Visceral Organ Inflammation (#1 Acute Hepatitis [elevated AST/ALT, 80%], #2 Acute Interstitial Nephritis [elevated Creatinine], and #3 Eosinophilic Myocarditis or Pneumonitis)—with intact mucosa and negative Nikolsky sign! Treat DRESS immediately by discontinuing the culprit drug and initiating Systemic Corticosteroids (slowly tapered over 6–12 weeks to prevent rebound flares).
Autoimmune & Metabolic Bullous Dermatoses: Histopathology & Immunofluorescence
Cutaneous blistering disorders are stratified by the anatomical plane of cleavage within the epidermis or dermoepidermal junction and the target adhesion complex destroyed by autoantibodies. Within the spinous and basal layers of the epidermis, keratinocytes are bound to one another by desmosomes (maculae adherentes) containing the calcium-dependent cadherins desmoglein-1 and desmoglein-3. Loss of desmosomal cohesion—termed acantholysis—produces fragile, superficial intraepidermal blisters that rupture easily upon lateral mechanical pressure (a positive Nikolsky sign). Conversely, basal keratinocytes are anchored to the underlying dermal basement membrane (lamina lucida/lamina densa) by hemidesmosomes containing BP180 (Type XVII collagen) and BP230. Autoimmune destruction of hemidesmosomes produces deep, subepidermal cleavage where the entire intact epidermis forms the roof of a thick-walled, tense bulla with a negative Nikolsky sign.
| Bullous Disorder | Target Antigen & Antibody Class | Histopathology & Direct Immunofluorescence (DIF) | Clinical Hallmarks & First-Line Therapy |
|---|---|---|---|
| Pemphigus Vulgaris | IgG autoantibodies against Desmoglein-3 (& Desmoglein-1) (desmosomes) | Suprabasal intraepidermal acantholysis; basal cells remain attached to basement membrane via intact hemidesmosomes ('row of tombstones'). DIF: Reticular (net-like / chicken-wire) intercellular IgG | Adults 40–60 yr: Flaccid bullae that rupture into painful erosions; Oral mucosa involved in >90% (often presenting sign); Positive Nikolsky sign. Rx: Rituximab (anti-CD20) + Systemic Corticosteroids (or azathioprine/mycophenolate) |
| Pemphigus Foliaceus | IgG autoantibodies against Desmoglein-1 ONLY (superficial granular layer) | Subcorneal/granular layer superficial acantholysis; intercellular IgG in upper epidermis | Shallow, crusted, scaly 'cornflake-like' erosions on seborrheic areas (scalp, face, chest, back); COMPLETELY SPARES ORAL MUCOSA (intact desmoglein-3 compensates in mucosa) |
| Bullous Pemphigoid | IgG autoantibodies against Hemidesmosomes (BP180 / Type XVII collagen & BP230) | Subepidermal blister cleavage with an eosinophil-rich dermal infiltrate; intact overlying epidermis. DIF: Linear IgG and C3 along the dermoepidermal junction | Elderly (>65 yr): Pruritic urticarial plaques evolving into large, tense bullae on flexural trunk/thighs; Oral mucosa spared; Negative Nikolsky sign. Rx: High-potency topical Clobetasol or oral Prednisone + Doxycycline |
| Dermatitis Herpetiformis | IgA antibodies against Epidermal Transglutaminase (eTG / TG3) cross-reacting with gut tissue transglutaminase | Subepidermal neutrophilic microabscesses at tips of dermal papillae. DIF: Granular IgA deposits at the tips of dermal papillae | Intensely pruritic, grouped ('herpetiform') vesicles and excoriations on extensor elbows, knees, and buttocks; pathognomonic for Celiac Disease. Rx: Gluten-free diet (definitive) + Dapsone (rapid acute relief) |
| Porphyria Cutanea Tarda (PCT) | Deficiency of hepatic Uroporphyrinogen Decarboxylase (UROD) (non-autoimmune heme defect) | Cell-poor subepidermal blister with festooning of dermal papillae; PAS-positive thickening of dermal capillary walls | Triggered by Hepatitis C, alcohol, estrogens, hemochromatosis (HFE): painless vesicles/bullae, milia, and scarring on sun-exposed dorsal hands + facial hypertrichosis + tea/red-wine colored urine (coral-red Wood lamp fluorescence). Rx: Therapeutic Phlebotomy or low-dose Hydroxychloroquine |
Pemphigus Vulgaris vs. Bullous Pemphigoid Clinical & Histologic Contrast
When approaching a blistering vignette, three rapid questions separate Pemphigus Vulgaris from Bullous Pemphigoid: (1) Are the blisters flaccid/ruptured or intact/tense? Pemphigus targets superficial desmosomes, so blisters rupture immediately into painful denuded erosions (Nikolsky-positive), whereas Bullous Pemphigoid targets deep hemidesmosomes, leaving a thick epidermal roof over tense bullae (Nikolsky-negative) that is much less likely to cause fatal fluid loss or sepsis. (2) Is the oral mucosa involved? Pemphigus Vulgaris almost always presents with painful oral erosions, whereas Bullous Pemphigoid typically spares the mouth. (3) What does Direct Immunofluorescence (DIF) show? Pemphigus shows a net-like (fishnet) intercellular IgG pattern around keratinocytes, whereas Bullous Pemphigoid shows a smooth linear IgG/C3 band along the basement membrane.
Bullous Pemphigoid Histopathology & Eosinophilic Infiltrate
On H&E skin biopsy of Bullous Pemphigoid, anti-BP180 IgG activates complement (C3a, C5a) at the lamina lucida, recruiting mast cells and numerous eosinophils to the dermoepidermal junction where proteolytic enzymes cleave the basement membrane zone without acantholysis. First-line management for localized or moderate disease in frail elderly patients is super-potent topical corticosteroids (0.05% clobetasol propionate cream), which achieves equivalent disease control to oral prednisone with significantly lower mortality.
Porphyria Cutanea Tarda & Dermatitis Herpetiformis Systemic Workup
In Porphyria Cutanea Tarda, accumulated uroporphyrins absorb 400 nm Soret-band ultraviolet light in dermal capillaries, generating reactive oxygen species that blister sun-exposed skin (dorsum of hands, forearms) while sparing covered skin. Unlike Acute Intermittent Porphyria (PBG deaminase deficiency, which causes neurovisceral abdominal pain without photosensitivity), PCT causes blistering photosensitivity WITHOUT abdominal pain. Workup mandates testing for Hepatitis C, HIV, and HFE hereditary hemochromatosis, followed by serial phlebotomy to reduce hepatic iron stores.
Papulosquamous & Eczematous Dermatoses: Psoriasis, Eczema, Lichen Planus & Pityriasis
Inflammatory papulosquamous and eczematous eruptions account for a large proportion of ambulatory dermatology board questions. Psoriasis Vulgaris is driven by Th1 and Th17 T-lymphocytes secreting TNF-α, IL-17, IL-22, and IL-23, which accelerate epidermal keratinocyte turnover from the normal 28-day maturation cycle to just 3–4 days. Conversely, Atopic Dermatitis (Eczema) stems from an epidermal skin barrier defect—classically a loss-of-function mutation in the filaggrin (FLG) gene—combined with a skewed Th2 immune response (IL-4, IL-13, elevated IgE). Recognizing distribution patterns (extensor elbows/knees in psoriasis vs. flexural antecubital/popliteal fossae in adult atopic dermatitis vs. flexural wrists/oral mucosa in lichen planus) allows immediate clinical diagnosis.
| Dermatosis | Pathophysiology & Associations | Classic Morphology & Anatomic Distribution | Histopathology & Stepwise Therapy |
|---|---|---|---|
| Psoriasis Vulgaris | Th1/Th17 cytokines (TNF-α, IL-17, IL-23) drive hyperproliferation; Guttate psoriasis triggered by Streptococcus pyogenes pharyngitis | Salmon-pink plaques with thick silvery scale on extensor elbows, knees, scalp, and gluteal cleft; Auspitz sign (pinpoint bleeding on scale removal), Koebner phenomenon, nail pitting | Histology: Acanthosis with elongated clubbed rete ridges, parakeratosis (retained nuclei in stratum corneum), thinned suprapapillary plates, Munro microabscesses (neutrophils in stratum corneum). Rx: Topical steroids + Calcipotriene (Vit D) → NB-UVB → Methotrexate or Anti-IL-17/IL-23/TNF |
| Atopic Dermatitis (Eczema) | Filaggrin (FLG) gene mutation (stratum corneum barrier defect) + Th2 cytokines (IL-4, IL-13, high IgE); Atopic triad: eczema, asthma, allergic rhinitis | Intense pruritus ('the itch that rashes'). Infants (<2 yr): erythematous weeping crusts on cheeks, scalp, and extensor limbs (spares diaper area). Older children/Adults: lichenified plaques in flexural creases (antecubital & popliteal fossae) | Histology: Acute spongiosis (intercellular epidermal edema); chronic acanthosis and hyperkeratosis. Complication: Eczema herpeticum (punched-out HSV vesicles) or S. aureus impetiginization. Rx: Thick emollients + Topical steroids / Tacrolimus → Dupilumab (anti-IL-4Rα) |
| Allergic Contact Dermatitis | Type IV (CD4+/CD8+ T-cell mediated) delayed hypersensitivity (48–72h) to haptens: Urushiol (poison ivy/oak), Nickel (jewelry, belt buckles), neomycin, fragrance | Geometric or linear erythematous vesicles and bullae strictly confined to the site of allergen contact | Dx: Epicutaneous patch testing. Rx: Allergen avoidance + high-potency topical steroids (or 2–3 week oral prednisone taper if >20% BSA or facial involvement) |
| Lichen Planus | CD8+ cytotoxic T-cell attack on basal keratinocytes; strongly associated with Chronic Hepatitis C Virus (HCV) and drugs (ACEi, thiazides) | The 6 Ps: Pruritic, Purple, Polygonal, Planar Papules and Plaques on flexural wrists, forearms, and ankles; lacy white Wickham striae on buccal mucosa/lesions; Koebner phenomenon | Histology: Hypergranulosis, 'sawtooth' band-like lymphocytic infiltrate at the dermoepidermal junction, apoptotic basal keratinocytes (Civatte / colloid bodies). Rx: High-potency topical steroids; screen for HCV |
| Pityriasis Rosea | Reactivation of Human Herpesvirus 6 or 7 (HHV-6 / HHV-7) | Begins with a solitary 2–5 cm salmon oval 'Herald patch' with a fine trailing inner collarette of scale, followed 1–2 weeks later by bilateral trunk papules aligned along skin tension lines ('Christmas-tree' pattern on back) | Self-limited eruption resolving spontaneously in 6–8 weeks; if rash involves palms/soles, always order an RPR to rule out Secondary Syphilis! Rx: Reassurance + topical pramoxine/antihistamines for itch |
| Seborrheic Dermatitis | Inflammatory response to lipophilic yeast Malassezia furfur (globosa) in sebaceous gland-rich regions; severe/explosive in HIV and Parkinson disease | Greasy, yellowish scales on erythematous base over the scalp (dandruff / infant 'cradle cap'), eyebrows, glabella, nasolabial folds, and central chest | Rx: Topical antifungal (Ketoconazole 2% cream/shampoo or selenium sulfide) + low-potency topical hydrocortisone for acute facial erythema |
Psoriasis Histopathology & Biologic Mechanism Matching
Biopsy of a psoriatic plaque reveals four classic histological features: (1) Acanthosis with regular downward elongation of the epidermal rete ridges; (2) thinning of the epidermis directly over dilated, tortuous dermal papillae capillaries—explaining the Auspitz sign (pinpoint bleeding when silvery scale is scraped off); (3) Parakeratosis (retention of flattened keratinocyte nuclei in the stratum corneum due to rapid transit) with loss of the stratum granulosum; and (4) Munro microabscesses (collections of neutrophils within the parakeratotic stratum corneum). Mild-to-moderate plaque psoriasis (<5% BSA) is managed with high-potency topical steroids plus vitamin D analogs (calcipotriene). Moderate-to-severe disease or concomitant psoriatic arthritis is treated with IL-17 inhibitors (secukinumab, ixekizumab), IL-23 inhibitors (guselkumab, risankizumab), IL-12/23 inhibitors (ustekinumab), or TNF-α inhibitors.
Atopic Dermatitis Complications: Eczema Herpeticum & Molluscum
Because filaggrin deficiency disrupts epidermal corneocyte cross-linking and raises skin pH, patients with Atopic Dermatitis have impaired innate antimicrobial peptide production and high rates of cutaneous superinfection. If an atopic child develops acute fever, malaise, and clusters of monomorphic, punched-out hemorrhagic vesicles and crusted erosions within eczematous skin, diagnose Eczema Herpeticum (Kaposi varicelliform eruption) caused by HSV-1/HSV-2 and initiate immediate systemic Acyclovir (with emergent ophthalmology evaluation if periorbital skin is involved).
Pityriasis Rosea vs. Secondary Syphilis & Tinea Corporis
The initial herald patch of Pityriasis Rosea is frequently misdiagnosed as Tinea corporis (ringworm); however, in pityriasis rosea the fine scale is located just inside the erythematous border (trailing collarette scale) and a KOH preparation is negative for hyphae. Once the generalized Christmas-tree eruption appears on the back and trunk, clinicians must verify that the palms and soles are spared; if palmoplantar macules or risk factors are present, obtain a serum RPR to exclude secondary syphilis.
Panniculitis & Hypersensitivity Spectrum: Erythema Nodosum, Erythema Multiforme & SJS/TEN
Board vignettes frequently test the distinction between Erythema Nodosum (a deep subcutaneous panniculitis), Erythema Multiforme (an infection-triggered targetoid hypersensitivity reaction), and the Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis (SJS/TEN) spectrum (a life-threatening drug-induced epidermal necrolysis). Although their names sound similar, their depths of pathology, etiologic triggers, mucosal involvement, and prognoses are entirely distinct. Specifically, Erythema Multiforme is almost always precipitated by Herpes Simplex Virus (HSV) or Mycoplasma pneumoniae and follows a benign self-limited course without skin sloughing, whereas SJS/TEN is >90% drug-induced via CD8+ cytotoxic T-cell release of granulysin, perforin, and granzyme B and carries a high mortality rate from sepsis and fluid loss.
| Condition | Primary Etiologic Triggers | Histopathology & Depth | Clinical Morphology & Mucosal / BSA Involvement | Management Protocol |
|---|---|---|---|---|
| Erythema Nodosum (EN) | NOUGS: No cause (idiopathic 50%), OCPs/pregnancy, Ulcerative colitis/Crohn, Granulomatous (Sarcoidosis, TB, Coccidioides, Histoplasma), Strep pyogenes | Delayed hypersensitivity septal panniculitis (inflammation of subcutaneous fat septa) without vasculitis | Painful, warm, erythematous subcutaneous nodules on the anterior shins bilaterally; never ulcerates; heals as bruise-like hyperpigmentation over 3–6 weeks | Treat underlying systemic cause (obtain CXR for Sarcoidosis/TB, ASO titer); leg elevation, NSAIDs (avoid NSAIDs if IBD is the trigger!), potassium iodide |
| Erythema Multiforme (EM) | Herpes Simplex Virus (HSV-1/2, #1 cause), Mycoplasma pneumoniae (especially in children); rarely drugs | Basal keratinocyte necrosis with dermal edema and interface lymphocytes | Symmetric typical 'target' (iris) lesions with 3 distinct concentric zones (dusky central blister/crust, pale edematous ring, outer erythematous halo) starting on acral palms/soles/dorsal hands; <10% BSA, Negative Nikolsky; EM minor (no mucosa) vs EM major (oral mucosa) | Symptomatic care (topical steroids, oral antihistamines); oral Valacyclovir suppression if recurrent HSV-associated EM |
| Stevens-Johnson Syndrome (SJS) | >90% Drugs (1–3 wk after start): Allopurinol, Lamotrigine, Carbamazepine, Phenytoin, Sulfonamides (TMP-SMX), Oxicam NSAIDs | Full-thickness epidermal necrosis due to CD8+ T-cell/NK-cell granulysin and Fas-FasL keratinocyte apoptosis | High fever/flu prodrome → coalescing dusky macules with < 10% BSA epidermal detachment, Positive Nikolsky sign, and severe hemorrhagic erosions of ≥2 mucosal sites (oral lips, conjunctivae/cornea, genitalia) | Stop culprit drug + Transfer to Burn ICU; aggressive IV fluid/electrolyte resuscitation, wound care, urgent ophthalmology consult (cyclosporine/etanercept/IVIG per institutional protocol) |
| Toxic Epidermal Necrolysis (TEN) | Same high-risk medications as SJS (strong association with HLA-B*15:02 for carbamazepine and HLA-B*58:01 for allopurinol in Asian populations) | Confluent full-thickness epidermal necrosis and subepidermal sheet separation | > 30% BSA sheet-like epidermal sloughing (10–30% BSA is SJS/TEN overlap) + severe multisite mucositis; high mortality (~25–30%) from S. aureus or Pseudomonas sepsis and hypovolemia | Burn ICU management; calculate SCORTEN severity score (age, malignancy, HR, BSA, BUN, glucose, bicarbonate) |
| DRESS Syndrome | Anticonvulsants (phenytoin, carbamazepine), allopurinol, vancomycin, minocycline, sulfonamides (long latency: 2–8 weeks after drug initiation!) | Perivascular lymphocytic and eosinophilic dermal infiltrate | Triad: Diffuse morbilliform eruption + facial edema + fever/lymphadenopathy + Marked Eosinophilia + Visceral organ inflammation (hepatitis #1, interstitial nephritis, myocarditis) | Stop culprit drug + Systemic Corticosteroids (gradual taper over 6–12 weeks to prevent rebound visceral flares) |
Erythema Nodosum Systemic Workup & Lofgren Syndrome
Erythema Nodosum represents a delayed-type hypersensitivity reaction in the interlobular fibrous septa of subcutaneous adipose tissue (septal panniculitis without vasculitis), presenting as tender, deep erythematous nodules over the anterior shins. Because up to half of cases reflect an underlying systemic disease, workup includes a chest radiograph, PPD/IGRA, throat culture/ASO titer, and bowel symptom review. On COMLEX and USMLE, the acute triad of Erythema Nodosum + Bilateral Hilar Lymphadenopathy + Migratory Polyarthralgias/Fever defines Löfgren syndrome, an acute benign variant of Sarcoidosis with an excellent prognosis that resolves with NSAIDs alone.
Erythema Multiforme vs. SJS/TEN: Target Lesions & Nikolsky Sign
Unlike SJS/TEN, Erythema Multiforme (EM) is NOT a precursor to TEN. EM presents with typical raised 3-zone target lesions distributed acrally on the palms, soles, and extensor extremities, lacks diffuse sheet-like epidermal detachment (Nikolsky sign is negative), and is most commonly triggered by HSV-1 or HSV-2 reactivation (often 7–10 days after a cold sore). Conversely, SJS/TEN presents with flat, atypical 2-zone dusky macules centered on the trunk and face that rapidly coalesce and slough off in sheets (Nikolsky-positive) alongside severe hemorrhagic crusting of the lips, pseudomembranous conjunctivitis (risk of symblepharon and blindness), and painful urethral/vaginal erosions.
Bullous Pemphigoid Direct Histopathology Reference
To contrast the full-thickness necrotic epidermis of SJS/TEN against autoimmune subepidermal blistering, examine the histopathology of Bullous Pemphigoid: the overlying epidermal roof remains completely viable and non-necrotic, separating cleanly at the dermoepidermal junction over a dermal papillae infiltrate rich in intact eosinophils.
Follicular & Sebaceous Disorders: Acne Vulgaris, Rosacea & Hidradenitis Suppurativa
Disorders of the pilosebaceous and apocrine follicular units—Acne Vulgaris, Rosacea, and Hidradenitis Suppurativa—require precise lesion classification to select targeted topical and systemic therapies. Acne Vulgaris pathogenesis involves four interlocking factors within the pilosebaceous unit: (1) follicular epidermal hyperproliferation and hyperkeratinization forming a keratin plug (microcomedo); (2) androgen-stimulated excess sebum production by sebaceous glands; (3) follicular colonization by the anaerobic diphtheroid Cutibacterium (formerly Propionibacterium) acnes, which hydrolyzes sebum triglycerides into pro-inflammatory free fatty acids; and (4) Toll-like receptor-2 (TLR-2) mediated perifollicular neutrophilic inflammation.
| Disorder / Subtype | Key Pathogenic & Clinical Features | First-Line Therapy | Refractory / Escalation Therapy & High-Yield Contraindications |
|---|---|---|---|
| Comedonal Acne (Non-inflammatory) | Open comedones (blackheads: oxidized melanin/lipids in dilated orifice) and closed comedones (whiteheads); no erythema or pustules | Topical Retinoid monotherapy (Tretinoin, Adapalene, Tazarotene) nightly to normalize follicular keratinization and comedolysis | Add salicylic acid or azelaic acid (azelaic acid also treats post-inflammatory hyperpigmentation and is safe in pregnancy); Tazarotene is Category X |
| Mild-to-Moderate Inflammatory Acne | Erythematous papules and pustules on face, chest, and upper back | Topical Retinoid + Benzoyl Peroxide (BPO) + Topical Antibiotic (Clindamycin or Erythromycin) | NEVER use topical or oral antibiotic monotherapy without Benzoyl Peroxide (BPO prevents C. acnes antibiotic resistance and bleaches fabric) |
| Moderate-to-Severe Papulopustular Acne | Widespread inflammatory papules, pustules, and early nodules | Topical Retinoid + BPO + Oral Tetracycline (Doxycycline or Minocycline) for ≤3–4 months | In female patients with hormonal/jawline flares or PCOS: add Combined Oral Contraceptive (OCP) + Spironolactone (androgen receptor antagonist) |
| Severe Nodulocystic / Scarring Acne | Deep, painful, coalescing dermal nodules, cysts, sinus tracts, and ice-pick/keloidal scarring | Oral Isotretinoin (13-cis-retinoic acid) for 15–20 weeks (atrophies sebaceous glands, reduces sebum by 90%) | iPLEDGE program: 2 negative pregnancy tests + 2 forms of contraception; check baseline/monthly fasting triglycerides & LFTs; CONTRAINDICATED with tetracyclines (pseudotumor cerebri!) |
| Rosacea (Erythematotelangiectatic, Papulopustular, Phymatous, Ocular) | Adults 30–60 yr (fair skin): central facial flushing triggered by heat/alcohol/spicy food, telangiectasias, papules/pustules with ZERO comedones, rhinophyma (bulbous nose) | Papulopustular: Topical Metronidazole, Azelaic acid, or Ivermectin (treats Demodex mites). Erythema: Topical Brimonidine (α2-agonist vasoconstrictor) | Refractory papulopustular or Ocular rosacea (blepharitis/keratitis): Oral Doxycycline; Avoid topical fluorinated steroids on the face (causes steroid rosacea/perioral dermatitis) |
| Hidradenitis Suppurativa (Acne Inversa) | Chronic occlusion of folliculopilosebaceous units in intertriginous apocrine areas (axillae, inguinal creases, perianal, inframammary); associated with obesity, smoking, metabolic syndrome | Hurley Stage I (solitary abscesses, no sinus tracts): Weight loss, smoking cessation, Topical Clindamycin 1%, intralesional triamcinolone | Hurley Stage II (recurrent abscesses, sinus tracts, 'double-ended' comedones, rope-like scars): Oral Doxycycline. Hurley Stage III (diffuse coalescing tracts): Adalimumab (anti-TNF-α) + wide surgical excision/unroofing |
Differentiating Acne Vulgaris, Rosacea & Steroid-Induced Acne
On board examinations, three features reliably distinguish Acne Vulgaris from Rosacea and Steroid-Induced Acne: (1) Acne Vulgaris peaks in adolescents, affects the face, chest, and back, and displays polymorphic lesions including open and closed comedones alongside papules, pustules, and nodules. (2) Rosacea affects adults aged 30–60, localizes strictly to the convex central third of the face (cheeks, nose, chin, forehead), exhibits persistent centrofacial erythema, telangiectasias, and trigger-induced flushing (alcohol, hot beverages, spicy foods, sunlight), and has NO comedones. (3) Steroid-Induced Acne (in patients taking systemic glucocorticoids or anabolic steroids) presents as a sudden, monomorphic eruption of uniform erythematous papules and pustules in the same stage of development across the chest, shoulders, and upper back without comedones.
Oral Isotretinoin Teratogenicity, iPLEDGE & Laboratory Monitoring
Isotretinoin is the only agent capable of inducing permanent remission in severe nodulocystic or scarring acne, acting by inducing sebocyte apoptosis, shrinking sebaceous glands, and normalizing follicular keratinization. Because retinoic acid disrupts cranial neural crest cell migration during weeks 3–8 of embryogenesis—causing craniofacial microtia/anotia, thymic aplasia, conotruncal cardiac defects, and hydrocephalus—all patients of childbearing potential must enroll in the FDA iPLEDGE registry, obtain two negative pregnancy tests before starting therapy, use two simultaneous forms of contraception from 1 month prior until 1 month after stopping therapy, and undergo monthly pregnancy, lipid panel (hypertriglyceridemia can precipitate acute pancreatitis), and hepatic transaminase monitoring.
Hidradenitis Suppurativa Pathophysiology & Hurley Staging
Despite its historical misnomer as a primary apocrine sweat gland inflammation, Hidradenitis Suppurativa (HS) initiates with follicular hyperkeratosis and occlusion of the terminal hair follicle in intertriginous skin, followed by follicular rupture into the dermis, secondary apocrine gland destruction, and chronic granulomatous inflammation. Unlike simple furuncles, HS lesions are sterile or polymicrobial, recur at the exact same anatomical sites (axillae, groin), form tombstone (double-headed) comedones, draining sinus tracts, and hypertrophic rope-like scars, and should NOT be managed with simple incision and drainage alone (which has a ~100% recurrence rate; perform punch debridement/deroofing instead).
OMM Board Correlate: Cutaneous Autonomics, Dermatome Mapping & Lymphatic Drainage in Inflammatory Dermatoses
Cutaneous Sympathetic Innervation: Unlike visceral organs that possess dual sympathetic and parasympathetic innervation, the skin (sweat glands, arrector pili muscles, and cutaneous arterioles) receives exclusively sympathetic innervation. Sympathetic efferents to the head and facial skin originate at T1–T4 and synapse in the superior cervical ganglion; upper extremity skin is supplied by T2–T8; trunk skin follows segmental thoracic levels; and lower extremity skin is innervated by T11–L2. In severe inflammatory or eczematous dermatoses, sympathetic tone regulates cutaneous blood flow, thermoregulation, and eccrine sweat secretion (which acts via postganglionic sympathetic cholinergic muscarinic receptors).
Viscerosomatic & Enterocutaneous Reflexes: Dermatologic conditions linked to gastrointestinal or hepatobiliary pathology—such as Dermatitis Herpetiformis (celiac disease, small intestine T9–T11), Erythema Nodosum / Pyoderma Gangrenosum (inflammatory bowel disease, T10–L2), and Lichen Planus / Porphyria Cutanea Tarda (chronic Hepatitis C, right-sided T6–T9)—produce predictable paraspinal somatic dysfunction and Chapman reflexes (liver at right 5th–6th ICS; small intestine at 8th–10th ICS).
OMT Protocol & Absolute Contraindications: Direct soft-tissue kneading, friction, or myofascial shear is strictly contraindicated over active bullous lesions (Pemphigus, SJS/TEN, where shear triggers Nikolsky epidermal detachment) and over psoriatic or lichen planus skin (where mechanical trauma induces new lesions via the Koebner phenomenon). Treat remotely by opening the thoracic inlet, doming the diaphragm, and performing gentle paraspinal rib raising to normalize sympathetic tone and lymphatic clearance.
- Never prescribe systemic oral corticosteroids (prednisone tapers) for routine plaque Psoriasis Vulgaris; abrupt withdrawal or tapering of systemic steroids frequently triggers life-threatening generalized pustular psoriasis or erythrodermic psoriasis.
- Never co-administer oral Isotretinoin (or high-dose Vitamin A) with Tetracycline-class antibiotics (doxycycline, minocycline) in a patient with acne vulgaris; both classes independently increase intracranial pressure and their combination precipitates Idiopathic Intracranial Hypertension (Pseudotumor Cerebri: headache, papilledema, CN VI palsy).
- In a patient presenting with intensely pruritic grouped vesicles and excoriations over the extensor elbows, knees, and buttocks (Dermatitis Herpetiformis), remember that it is 100% associated with Celiac Disease; acute pruritus responds within 48 hours to Dapsone (screen for G6PD deficiency first!), while long-term remission requires a strict lifelong gluten-free diet.
- In Stevens-Johnson Syndrome (SJS) and Toxic Epidermal Necrolysis (TEN), immediately discontinue the offending medication (allopurinol, lamotrigine, carbamazepine, phenytoin, sulfonamides, oxicam NSAIDs) and transfer the patient to a specialized Burn Intensive Care Unit for aggressive fluid/electrolyte replacement and wound care; prophylactic systemic antibiotics are contraindicated unless clinical sepsis develops.
- Always screen patients newly diagnosed with Porphyria Cutanea Tarda (blisters and milia on sun-exposed dorsal hands, facial hypertrichosis, elevated urine uroporphyrins) or Lichen Planus (pruritic purple polygonal planar papules with Wickham striae) for chronic Hepatitis C virus (HCV) infection.
- In Pemphigus Vulgaris, painful oral mucosal ulcerations precede cutaneous flaccid bullae in the majority of patients; because desmoglein-1 is expressed throughout the epidermis while desmoglein-3 is concentrated in basal/mucosal layers, mucosal-dominant Pemphigus Vulgaris has anti-desmoglein-3 antibodies, whereas Pemphigus Foliaceus (anti-desmoglein-1 only) causes superficial crusted erosions that completely spare the oral mucosa.