THE USMLE TIMES - #100
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Welcome to issue #100 of The USMLE Times! We’ve now successfully written one hundred weeks in a row! Thank you to all of our supporters—new and old!
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Here’s what’s on tap for issue #100 of the USMLE Times:
Question of the Week (Foundational Renal Physiology)
This week’s video training (The King of Study Techniques)
Question deep-dive & breakdown
Let’s dive in!
A 35-year-old woman presents to her primary care physician complaining of fatigue and muscle cramps over the past few weeks. She reports that she has been taking naproxen for her chronic lower back pain. Her vital signs are within normal limits, and physical examination reveals no significant findings. Laboratory results show a serum creatinine level of 1.6 mg/dL, blood urea nitrogen (BUN) of 30 mg/dL, and potassium level of 5.6 mEq/L. Which of the following mechanisms of action is the most likely cause of the patient’s acute kidney injury?
A. Inhibition of carbonic anhydrase
B. Inhibition of the sodium-potassium pump in the basolateral membrane
C. Inhibition of the sodium-potassium-chloride cotransporter
D. Inhibition of the sodium-chloride cotransporter
E. Inhibition of prostaglandin synthesis
The answer & question breakdown is at the bottom of the post 👇🏼
🔗 LINKS TO RECENT POSTS
USMLE Times Issue #99 - READ HERE
USMLE Practice Question #86 - TRY IT HERE
USMLE Practice Question #85 - TRY IT HERE
This week’s recommended video 👇🏼
ANSWER + QUESTION BREAKDOWN
The mental model used to answer this question comes from our detailed test-taking skills masterclass (check it out if you want to elevate your USMLE test-taking skills) Here’s how to think through this question:
Step 1. Read the last line to determine the question: “Which of the following mechanisms of action is the most likely cause of the patient’s acute kidney injury?”
Step 2. Determine if this is a first-, second-, or third-order question. It is 3rd order. 1st: Diagnose the problem; 2nd: Correctly identify the cause; 3rd: Identify the mechanism underlying the likely cause.
Step 3. Read and understand the vignette, then ask yourself: “What’s the most likely underlying cause & mechanism underlying this patient’s symptoms?”
Step 4. Look at the answer choices and select the option most closely resembling your thoughts from “Step 3” above.
GENERAL ANALYSIS
The 35-year-old woman in this case presents with renal dysfunction (elevated serum creatinine and BUN) and hyperkalemia (elevated potassium level). She has been using naproxen, a nonsteroidal anti-inflammatory drug, for chronic lower back pain. NSAIDs are known to cause acute kidney injury, particularly in susceptible individuals, by impairing renal blood flow.
ANSWER CHOICES:
ANSWER CHOICE A: Inhibition of carbonic anhydrase
Explanation: Carbonic anhydrase inhibition does not explain NSAID-induced AKI. Acetazolamide is used for conditions like glaucoma, altitude sickness, and metabolic alkalosis but is not involved in prostaglandin synthesis or renal blood flow regulation.
ANSWER CHOICE B: Inhibition of the sodium-potassium pump in the basolateral membrane
Explanation: NSAIDs do not inhibit this pump. Drugs that affect this pump include cardiac glycosides like digoxin, which inhibit the sodium-potassium ATPase in cardiac myocytes to increase intracellular calcium levels and enhance contractility.
ANSWER CHOICE C: Inhibition of the sodium-potassium-chloride cotransporter
Explanation: NSAIDs do not act on this cotransporter. Loop diuretics are used for conditions like heart failure and edema but are not involved in prostaglandin synthesis or afferent arteriole vasodilation.
ANSWER CHOICE D: Inhibition of the sodium-chloride cotransporter
Explanation: NSAIDs do not affect NCC. Thiazide diuretics are commonly used for hypertension but are not involved in prostaglandin synthesis or renal blood flow regulation.
ANSWER CHOICE E: Inhibition of prostaglandin synthesis
Explanation: By inhibiting prostaglandin synthesis, NSAIDs cause vasoconstriction of the afferent arterioles, reducing renal perfusion and glomerular filtration rate. This can lead to acute kidney injury, especially in patients with predisposing factors like dehydration or pre-existing renal impairment.
THE VERDICT…
NSAIDs like naproxen inhibit the COX enzyme, which is responsible for converting arachidonic acid into prostaglandins. Prostaglandins (especially PGE2 and PGI2) play a crucial role in maintaining renal blood flow by dilating the afferent arterioles of the kidneys. This dilation ensures adequate glomerular filtration, particularly in situations of reduced effective circulating volume like dehydration and heart failure.
The patient’s elevated serum creatinine and BUN levels suggest reduced GFR, and hyperkalemia indicates impaired renal excretion of potassium. These findings are consistent with NSAID-induced AKI.
FINAL ANSWER: E. Inhibition of prostaglandin synthesis
That’s it for issue #100 of The USMLE Times!
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