ITB Principles for the USMLE, COMLEX, and Shelf Exams: Follow the Pathophysiology Part 1

ITB Principles for the USMLE, COMLEX, and Shelf Exams: Follow the Pathophysiology Part 1

Courtesy of Julie Falk

Courtesy of Julie Falk

In my article “How to Use a Pea Plant to Increase your USMLE, COMLEX and Shelf Exam Scores” over at SDN, I promised to post a companion piece dissecting the question mentioned in the body of that article. So here is part one (look for part 2 on Wednesday) of the companion piece. Here I’m going to teach you the following principle: when two or more answer choices can both lead to the similar (or same) “result”, you should pick the one that results from a derangement of a physiologic process.

Obviously, your first step in approaching any multiple choice question is to read the stem, or vignette, the part of the question that tells the patient’s story.

A newborn infant is examined in the delivery room and found to have ambiguous genitalia. Non-invasive prenatal testing was performed at 11 weeks gestational age and the results showed a karyotype consistent with a female fetus. Physical examination of the infant shows clitoromegaly and partial fusion of the labial folds. The infant appears otherwise well with normal vital signs.

One advantage for the test taker with respect to multiple choice questions (MCQs) is that the answer is actually right there in the vignette. You just have to recognize it. And remember, the majority (over 90%) of questions on the COMLEX, USMLE, shelf exams and most medically-related “board” exams are going to be single best answer questions. So there is likely to be one answer that is 100% correct and the rest are going to be wholly wrong—or annoyingly and quite often—less than 100% true.

Before looking at the answers, or even the question you’re being asked, you have to note the essentials of the stem. This should become automatic, allowing you to immediately hit on the critical info that will get you the point for that question. Good question writers give you just enough info to arrive at the correct answer and just enough to rule out the incorrect ones. This trains your mind to think clinically.

For the most part, the essential aspects of the vignette related to the correct answer are the demographic info; the chief complaint/presenting issue; and pertinent positive historical details, physical exam findings, or objective studies like labs and imaging modalities. So let’s break it down.

Demographic: newborn infant. 

Presenting Issue: ambiguous genitalia. 

Objective studies: karyotype consistent with a female fetus. 

Pertinent positives: clitoromegaly and partial fusion of the labial folds.

Your goal from dissecting out this info should be to come up with an idea in your head about what you’re dealing with. Don’t look at the answer choices yet or even the interrogatory/lead-in (the question that’s actually being asked). This only has the potential to confuse you. You won’t always be able to come up with an exact diagnosis at this point but if your mind automatically jumps to a particular diagnosis, it’s likely that this framework is the correct one, coming from the deep recesses of what you’ve learned. When you read those four items, ask yourself what immediately comes to mind? What diagnoses fit this description?

You know you’re dealing with a female infant with ambiguous genitalia. So the differential in your mind will hopefully match the answer choices you’ll encounter as you read through the rest of the MCQ.

But first, you have to tackle the interrogatory, or lead-in. If you’ve followed the above process, you already have an idea of what’s going to be asked. If you’ve come to the end of this process and concluded that the most likely diagnosis is congenital adrenal hyperplasia (CAH), congratulations! You’re pretty much there. If you didn’t arrive at that conclusion. No worries. Read on.

Next you read the interrogatory. Maybe the question is looking for the mechanism of the ambiguous genitalia (more likely with Step 1/Level 1 content), maybe it’s straight up diagnosis (the focus of Step/Level 2), or maybe it’s how to manage the case presented (the favorite of Step/Level 3).

Which of the following is the most likely mechanism involved?

But wait. I just told you to fix the diagnosis (or differential) from the vignette in your mind. But this isn’t asking you “what is the most likely diagnosis?” Don’t panic. The vast majority of the time, you are going to have to know the diagnosis the vignette is describing before you can answer any question you’re presented. It’s the foundation. But here’s where the real fun—er, work?—begins.

If you didn’t automatically hit upon CAH as the diagnosis, begin to ask yourself what you know about female infants with ambiguous genitalia. Begin your self-interrogation. Probe your memory for anything you know about the development process of normal external genital. What conditions are necessary for the process to go smoothly? If you’re stuck, start with what’s already given to you in the vignette. Getting to the root of your knowledge may stimulate your memory of some key point that will help you arrive at the correct answer.

Now look at the question in its entirety.

A newborn infant is examined in the delivery room and found to have ambiguous genitalia. Non-invasive prenatal testing was performed at 11 weeks gestational age and the results showed a karyotype consistent with a female fetus. Physical examination of the infant shows clitoromegaly and partial fusion of the labial folds. The infant appears otherwise well with normal vital signs. Which of the following is the most likely mechanism involved?

A. Androgen receptor defect

B. Complete gonadal dysgenesis

C. Deficiency of 21-hydroxylase

D. Ovarian failure

E. Prenatal androgen exposure

You know the infant is a female with a normal karyotype. So you also know you’re dealing with X,X genetics. Next time (check back on Wednesday), we’ll go through each answer choice using the same memory interrogation process and see what you can rule out based on this knowledge. Keep asking yourself questions until you’re able to rule in or rule out a particular answer choice.

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