DIP Episode 69 - USMLE Anatomy Series 2

Okay, welcome. My name is Divine. I am a PGY1 resident that is ultimately going into radiology and this is the 69th episode of the Divine Intervention Podcasts. This is actually a continuation from episode 44. So this will be the USMLI Anatomy Series part two. Basically I'm gonna make like a series relating to anatomy as relevant to the USMLI exams, especially the USMLI Step 1 exam. This may also be barely helpful for surgery shelf, but I'll say mostly for the USMLI Step 1 exam. So let's jump right in. So first question and 18-year-old female presents to the ED with a two-day history of difficulty chewing. Okay, that's not good. She leaves in connectic gut. Okay, and recently noticed a bull's eye rash on her right lower extremity. Okay, physical exam is notable for an asymmetric smile. Ding ding ding. Okay, so this question is saying what's your diagnosis? What's the pathophysiology? What's the potential reason why she cannot chew or let's assume this was a baby question like the baby cannot suckle and what are the treatment strategies? So this first patient because again like I said, for these podcasts I try to make key scenarios of like classic exam presentations to introduce concepts. I think it just makes it easier to remember things that way. You probably heard me give a more extended discussion on why I think key space learning is very useful if you're listening to my central dogma of studying a podcast. So this patient obviously has Bell's policy.

Okay, remember Bell's policy is a cranial nerve seven injury. Okay, usually it's like swelling around a cranial nerve seven, right? And you can have many theologies, right? So you can arise from HSV, you can arise from sarcoidosis, you can actually arise from Lyme disease. In fact, if you see a bilateral Bell 's policy on the USMLE exam, you really want to think about Lyme disease. Please don't forget, right? Lyme disease is caused by Borrelia Bokdofri that's carried by the Exodus tick, right? And the skin lesion I described in the question is the erythema chronicomychras, right? So like the bulls-eye rash. So if you have problems with cranial nerve seven, right? If you remember from anatomy, whenever you took your anatomy course in your first year, I remember that cranial nerve seven innervates the muscles of facial expression. And one of those muscles that, yeah, it helps with facial expression to an extent, but it actually helps your muscles of mass-dication to actually bring me a pretty significant extent is your boxinator muscle. Your boxinator muscle, in fact, let me give you a nice way to remember this because when you group things, it's easier to remember. The thing is your boxinator muscle is a muscle of facial expression, but your boxinator muscle also assists your muscles of mass-dication. Your muscles of mass-dication, like they do their job, but they get some help from the boxinator muscle.

The boxinator muscle is not a muscle of mass-dication, but it assists your muscles of mass-dication. And remember that your muscles of facial expression are innervated by the facial nerve, cranial seven. Your muscles of mass-dication are innervated by your mandibola nerve. That's the third branch of the trigeminal nerve. So if you have a boxinator muscle that has two lives, basically, it acts as a muscle of facial expression and also assists your muscles of mass-dication, it should then help you remember that it receives innervation from the facial nerve. It gets mortal innervation from the facial nerve, but it actually gets sensory innervation from the trigeminal nerve. In fact, the nerves are the bookon nerves. There's a bookon nerve, cranial seven branch, and there's a bookon nerve, cranial five branch. The bookon nerve, cranial five branch, does sensory innervation for the boxinator muscle. The bookon nerve, cranial seven branch, does motor innervation for the boxinator muscle. Really, for bell spalsy, you give steroids, you can give HSV, I mean, you can give, no, don't give people HSV. You can give a cyclover, it may help in certain cases of a bell spalsy. If you know that the bell spalsy is rising from, I don't know, like, Lyme disease as is this case, you want to treat for Lyme, right? So, usually you give like septal axon or you can give like doxycycline, okay? So this is one of those weird things you want to keep at the back of your mind.

And for the most part, bell spalsy results in most people. That's like the natural history of the disease. So, next question, a three-year-old male mistakenly bites the tip of his tongue while enjoying a nice meal from McDonald's, okay? He cries in severe pain as his mom consoles him. What is the cranial nerve that is associated with this toddler being able to experience the sensation of pain, okay? So, this is basically, right? So, the tip of the tongue, right? That is at least the part of the anterior to the third of the tongue, right? And we know that pain sensation, right? So, general sensation from the anterior to third of the tongue is undertaken by the trigeminal nerve, okay? By the lingua nerve, okay? So, that's just a weird thing you want to know. Generosity sensation for the anterior to third of the tongue, like pain, pain, pain, pain, temperature, whatever, for the anterior to third of the tongue is trigeminal nerve territory, but and that's the lingua nerve if you want to be a little more specific, but if you're talking about special taste sensation from the anterior to third of the tongue, that is facial nerve cranial seven territory. You may say, oh, divine, you mentioned this in the first anatomy series podcast. Yeah, I'm repeating again because it's very high you to know for exams. It's one of those things that even you may be taking step three and see tested, okay? So, don't bluff this piece of information.

So, next question, what are the three-middle ear bones? This is a relatively easy question, but the second question is the meat for step one, right? So, how do they the fair with respect to brain kill, or pharyngeal arch derivation and innovation? Okay, so, the three-middle ear bones, they're pretty easy, right? So, malias, incas, and stapes, right? So, MIS, malias, incas, and stapes. The thing is, the malias and the incas. So, the first two, they actually arise from the first brain kill arch, okay? And the thing is, that first brain kill arch is innervaded by the second and third branches of the trigeminal nerve, right? So, like the maxillary and mandible nerves. The stapes is actually innervaded by cranial nerve seven, the facial nerve, okay? In fact, if you remember from anatomy, if you sort of like scroll back in your memory bank to like your first few weeks of anatomy in med school, you probably remember nerve to the stapidius. The nerve to the stapidius is a branch of the facial nerve, which is cranial nerve seven, okay? So, you may say like, define, why do you think this is important? Well, this is important because your friends at the MBM will say, oh, if you remember the three bones like malias, incas, stapes, so, oh, they must all have the same innervation. No, they don't, okay?

Malias, incas, are innervaded by the trig, by branches of the trigeminal nerve, the maxillary and mandible branches, and the stapes muscle is, I mean, the stapes bone is innervaded by the facial nerve, okay? By a branch of the facial nerve. So, what are some other high-yale things to remember, right? Well, I mean like, malias has an M as the first letter, right? The thing is in general M structures, so this is a nice demonic M structures tend to come from the first arch, right? So, like your muscles of massication, right? They are all innervaded by the trigeminal nerve. The masseter is one of those muscles of massication, right? It's innervaded by the trigeminal nerve, comes from the first arch, the malu-highoid muscle, right? It comes from the first arch. The malias of the ear comes from the first arch, right? Contrast that with the second arch, the loves to innervate and like S structures, right? So, or you can say like the second arch gives rise to S structures, right? So, like the stapes muscle, I mean the stapes bone, right? The stupidius muscle, right? Your stylohyoid, right? So, remember I said my low-highoid is an M structure, that's first arch, stylohyoid, that's an S structure, that's second arch, okay? So, that's an easy way to remember that.

And then I know the bizarre thing that they love to do on the USML Es, relating to the first arch versus the second arch is to see if you can remember or recall what part of the digastric muscle is innervaded or comes from the first arch versus the second arch. Here's the key thing you want to remember. The digastric muscle has an anterior belly and a posterior belly. The anterior belly of the digastric muscle comes from the first brinkial arch. The posterior belly of the digastric muscle comes from the second brinkial arch. So, how do you let me see divine? How do I remember all these details? Well, remember, anterior belly has an A, posterior belly has a P, A comes before P in the alphabet. And the first arch, number one, comes before the second arch, number two. posterior belly of the digastric goes with second arch and anterior belly of the digastric goes with the first arch. So, I hope that helps you out there. So, let's jump to the next question. So, the next question says, a twin-three-year-old female just broke up with her boyfriend of three months. Oh, that's sad. She's brought to the ED by her roommate who noticed blood on her bed. Physical exam is notable for a superficial left palm laceration. She has difficulty flexing the PIP and DIP of her left index finger. What's the diagnosis and what's the pathophysiology? So, for this, right? So, this patient obviously has borderline personality disorder. In fact, I'll just tell you this.

If you ever see a female on any NV Me exam, so a female that slashes her wrists and has like ten-nose relationships like, oh, like the break up repeatedly and all that stuff, think about borderline personality disorder. And don't forget that those people use splitting as a defense mechanism, okay? So, since this patient cannot flex her PIP and DIP, think of this as injury to the flexor digital and profoundus muscle, okay? So, flexor digital and profoundus muscle injury will cause problems with flexing your PI Ps and your DI Ps. And the thing is, when you're taking the USM Ls, your friends at the NV Me will try to trick you by putting like flexor digital and profoundus as an answer. When you will also try to put like median nerve as an answer, you'll try to put on and on as an answer, resist the temptation to pick any of those answers. Because it so happens that the flexor digital and profoundus muscle actually gets innervation from both nerves. The median and the oner nerves innovate the flexor digital and profoundus muscle. In fact, the median nerve does the lateral side of the muscle. The oner nerve does the medial side of the muscle. And I mean, you may say, how do I remember that? I mean, just hold out your palm, right? Remember that your oner artery, your oner nerve sort of runs along your medial if you Indian atomic opposition runs along your medial forearm, your medial nerve runs along your lateral forearm, right?

So, it should make sense that the medial side of the flexor digital and profoundus muscle gets innervation from the oner nerve with the lateral side of the flexor digital and profoundus muscle gets innervation from the median nerve. Okay? So again, very high up to know that. So let's jump to the next question. So 16 year old high school football player is hit by an opponent during a game and immediately begins to complain of severe right shoulder pain. He has difficulty with lateral rotation of his right arm. What is the most likely finding on imaging? What is the lesion nerve? I would encourage you to not take actually don't take any MBM exam, at least like your surgery shelf or step one step two or step three, if you do not know the answer to this question. So this patient basically has one of two things, right? They either have like an anterior dislocation of the shoulder or they have a surgical neck fracture of the humerus, right? So basically, if you cannot lateral rotate your arm, that means you screwed up your axillary nerve, okay? Your axillary nerve, remember your axillary nerve is C5 C6, very high up to know that. Your axillary nerve controls lateral rotation of your arm, okay? It also controls A Bduction of your arm and it also controls sensation on your lateral arm. Remember, your arm is basically the part between your shoulder and your elbow, your forearms between your elbow and your wrist.

So they can give you any of those lesions and then you need to trace it back to an axillary nerve injury and then you need to trace that back to a C5 C6 nerve root, I need to trace that back to an anterior dislocation of the shoulder or, or what is it called? Or a surgical neck fracture of the humerus and it's actually kind of high up to know that the most common direction of shoulder dislocation is anterior dislocation, okay? Contrast that with your hip, your hip usually dislocates in a posterior direction. Again, I promise you all these things I'm saying a very high up to know. I'm be saying them if they were not high up in the first place. And if you ever saw a posterior shoulder dislocation on your USMLH, you want to connect that with either a person that recently had a seizure or a person that was recently electrocuted or struck by lightning or something ridiculous, okay? But posterior shoulder dislocations are very rare. If you want that to play odds and your shoulder dislocations are the most common. So let's jump to the next question. So the next question says a 16 year old high school football player is hit by an opponent during a game and immediately begins to complete a severe right knee pain. On physical exam there is excessive laxity noticed when the femur is pulled anteriorly relative to the t-beer. So what's your diagnosis and what's the common mechanism of injury?

So I know as I was reading this question the very first thing that probably popped into your mind that you wanted to see out loud was oh this is an ACL tear. Well bad news. This is not an ACL tear. And I can imagine you're probably pinching yourself as you realize that now. So this is something that your friends on the NBA may love to do to you on exams, right? So this patient actually has a PCL tear right. So the thing is the people at the NBA mirror they will try to mess with your head on the exam by sort of being weird with the terminology right. So this is where you have to be careful and I will give you an easy way to remember like the direct reality of things at the end. But the thing is your posterior crochet ligament actually prevents excessive anterior displacement of the femur relative to the t-beer. Okay your PCL presents prevents excessive anterior displacement of the femur relative to the t-beer. But you can also say that the PCL prevents excessive posterior displacement of the t-beer relative to the femur. I know it's annoying. So try to keep it straight. In fact I'll tell you this. If you want to say like oh A in ACL or P in PCL like the A ACL anterior P PCL posterior those terms if you want to keep everything sort of uniform relate to the displacement of the t-beer relative to the femur. Okay so if you remember that oh like an AC if you have a PCL tear you have excessive posterior displacement of the t-beer relative to the femur.

If you just remember that statement then you're good to go because if they present that as the scenario in the exam you know it's a PCL tear. But if they present like if they give you any exam question where they talk about the displacement of the femur relative to the t-beer then know to pick the opposite right. So if they say oh this person has excessive anterior displacement of the femur relative to the t-beer. You're like oh wait even if it's saying anterior I need to flip it in my mind and make it posterior because it doesn't line up with the story I already know. So I hope that makes sense that's very high order to know right. So again it will be nice if they use like if they said oh this person cannot have has I mean this person has excessive posterior displacement of the t-beer relative to the femur and then the answer is PCL tear and everything will be good and everyone will be all happy. But the thing is if that was the case everyone will get a hundred percent on the USMLE's right which is not ideal from a test in a standpoint. So try to remember one thing and then be able to derive other knowledge from that single thing okay and usually the way you actually damage the PCL is if you have like a direct blow to a flexed knee right. So say for example a person is in a motor vehicle accident and their knees running to the dashboard that is one relatively common mechanism behind a behind a PCL tear right. So again if you want to let's practice this again right.

So if a patient has an ACL tear right. They would have excessive anterior displacement of the t-beer relative to the femur okay or you can see that they will have excessive posterior displacement of the femur relative to the t-beer okay. So make sure you understand what I just said okay if you don't rewind this and listen to it again. So I think yeah I think I've made that point strongly enough please don't get that wrong gonna test. You just don't have those things you get wrong you like you just really feel bad after getting it wrong because it's something that you can get easily if you just take like two minutes to like understand it. So the next question a ballet performer presents to a PCP at the end of dance season with difficulty walking. During a neurologic exam she has difficulty raising her left foot to the ceiling okay. So she has difficulty raising her left foot to the ceiling so basically she has like a foot drop if you may okay. Low extremity MR imaging right. So like a low extremity MRI reveals compression of a nerve. So what's the diagnosis and what's the pathophysiology right. So the thing is this patient has a common perinoid nerve injury. The thing that likely happened is that the perinoid nerve for this patient was compressed at the fibula neck. I mean it's pretty common in ballet dancers right.

So people that are like just chronically bedbound they can compress that common perinoid nerve at the fibula neck and if you compress the common perinoid nerve actually some resources refer to it as the fibular nerve. If you compress that nerve you have trouble you basically have foot drop right and I mean there's a nice and one you can first eat that if your perinoid nerve is going you your foot will be dropped right. So like DRO PPED right. So the P one of the P's usually to remember that it's a perinoid nerve injury and then the E after P remember that you have trouble everting your foot and then the D which is the last letter remember you have trouble or see flexing your foot. That's why those people will not be able to read that's why this ballet dancer cannot raise her foot towards the ceiling. And I mean if they want to make the question a little more difficult they can describe like sensory losses. Basically if you scrub your common perinoid nerve you have like a loss of sensation between your first and second toes. You also have loss of sensation actually along your lateral leg and on the dorsum of your foot. So the dorsum of your foot is just basically like the skyward facing part of your foot. Okay remember that skyward facing part of your foot. Dorsum dorsiflexion right. So just to tie all those things together. So if you have a so I guess sort of to summarize if you have a common perinoid nerve injury you have foot drop okay.

So perinoid nerve injury trouble everting your foot trouble dorsiflexing your foot loss of sensation over the dorsum of the foot right and you also lose sensation over your lateral leg and between your first two toes. So I hope that clears that up for you. Okay so next question and 89 year old this is actually the last question. So an 89 year old female presents to a PCP for a full of visits two weeks after having hip replacement surgery. Physical exam is notable for drooping of the left hip. So basically the left hip sucks down when the patient is made to take several forward steps. What's your diagnosis? What's the legion nerve? What are the legion muscles? What are the associated nerve roots? And what's the damage? What is the damaged site in this patient? It's a very high-yield scenario to know for your for your exam. So basically this patient has something we call a trendellan bird gate. Okay. Classically on exams this can arise if a person has like like hip replacement surgery or it can arise if a person has like an L5 radicalopathy. Basically radicalopathy B that's like the 5 term for nerve recingering. Okay so the thing is the nerve that we are concerned about in this case is the superior gluteal nerve. The superior gluteal nerve comes from L4 L5 and S1. Okay the thing is if you lesion the superior gluteal nerve your hip ab doctors. So your hip AB doctors will not work. Okay and I mean you should probably remember what your hip ab doctors are.

Right so like those are muscles like gluteus medias gluteus minimus and tens of fascial ade. The one you'll probably see tested on your exam is Gluteus medius. Okay that's like the big one to remember for exams. Gluteus like medius minimus tens of fascial ade those are your hip ab doctors. Okay if your hip ab doctors do not work. So if your hip ab doctors do not work right you will not be able to basically abdoct your trunk to keep it steady when you're walking at least during like the stance phase of the gator of the gator cycle. Okay so if your ab doctors don't work you cannot mean basically you cannot maintain steady hips when you're walking. Right so here's where you kind of need to exercise caution though in fact that's why I added a question at the end that said what's the damaged side. Okay the thing is the superior gluteal nerve that is lesion is always on the control lateral side to the droopy hip. Okay so for example right so this patient has a left-sided droopy hip right so basically their hip sucks down on the left that means they have a right superior gluteal nerve lesion right so they likely had like a right hip replacement but I mean if I said oh they had a right hip replacement I know my students are good test-takers you probably get it right off the bat right so that's why I did not specify a site so sort of make you think.

So that's why they have a left-sided droopy hip okay so in this patient actually the side that drops is actually normal because if you actually screw up both superior gluteal nerves you actually have something called a wadling gate like basically like you have both hips sort of dripping just watch a You Tube video it's a pretty striking when you see it and I guess while we're on this topic even if I did not put this in the queue stem I'll just go ahead and talk about it right so you may have like I guess like a peripherally related question where they may talk about a patient who has like difficulty standing from a stooped position right and let's assume this patient also again recently had a hip replacement in this case think about an inferior gluteal nerve injury okay the thing is unlike the superior gluteal nerve that innervates gluteus medius many more sentence of fascia lada the inferior gluteal nerve innervates the gluteus maximus muscle basically that gluteus maximus is like your big buttock muscle right so if you fill your buttocks and like it's like big you have like a larger gluteus maximus basically the main function of that muscle is to help you keep your trunk erect right so if you stoop down and then you rise if you want to sort of stand erect you need your gluteus maximus muscle to make that happen so if you lesion the gluteus maximus muscle or you lesion the inferior gluteal nerve you don't have your ability you don't have the ability to do this in fact I mean you may notice right many animals they have to sort of be on all fours many animals cannot stand erect because many of them either have no gluteus maximus muscle or it's like severely underdeveloped right that's why like many animals have like very like flatter buttocks if you may so I think that's all I have to discuss today I'm intentionally making this podcast shot so that is not overwhelming b

ecause anatomy has has a way of becoming of a woman are really fast and before I sign off I just want to mention like I mentioned in other podcasts I offer tutoring for the USML is step one step two CK two CS and step three and also the medicine in training exam and at some point in the near future the internal medicine aboard exams and also prepare applications and prepare people for interviews for med school and residency so if you need help with any of those things please feel free to reach out anytime the email is divine intervention podcasts at gmail.com it's podcasts with an S at the end just reach out and we can talk about pricing and things of that nature so I wish all the best I am certainly glad the Lakers won your game last night and I will see you in the next podcast God bless and thank you

M structures (Masseter, Maxilla) generally come from the 1st arch; S structures (Stylohyoid) generally come from the 2nd arch.

Lyme disease (Borrelia burgdorferi).

Foot drop and loss of sensation over the dorsum of the foot and lateral leg.

The digastric muscle (Anterior belly = 1st arch; Posterior belly = 2nd arch).

To help keep the trunk erect by extending the hip, especially when rising from a stooped position.

The flexor digitorum profundus (Median supplies lateral side; Ulnar supplies medial side).

Lingual nerve (a branch of CN V3/Trigeminal Nerve).

Facial nerve (CN VII) via the chorda tympani.

L4, L5, S1; Innervates Gluteus Medius/Minimus, responsible for hip abduction.

Axillary nerve (C5-C6).

Direct blow to a flexed knee (e.g., dashboard injury).

Gluteus Medius/Minimus; Trendelenburg gait (on the contralateral side of the droopy hip).

D=Dorsiflexion difficulty (foot drop); R=Eversion difficulty; O=Loss of sensation over dorsum/lateral leg; P=Peroneal nerve.

Lingual nerve (a branch of CN V3/Trigeminal Nerve).

Facial nerve (CN VII) via the chorda tympani.

L4, L5, S1; Innervates Gluteus Medius/Minimus, responsible for hip abduction.

Axillary nerve (C5-C6).

Direct blow to a flexed knee (e.g., dashboard injury).

Gluteus Medius/Minimus; Trendelenburg gait (on the contralateral side of the droopy hip).

D=Dorsiflexion difficulty (foot drop); R=Eversion difficulty; O=Loss of sensation over dorsum/lateral leg; P=Peroneal nerve.