USMLE PRACTICE QUESTION #70
TOPIC: Vascular physiology, pathology, and pharmacology
Today’s question will test your understanding of vascular physiology, pathology, and pharmacology.
I’m Paul Ciurysek, MD, and this is The USMLE Times: An independent, subscriber-supported newsletter designed to help you survive medical school, pass your USMLE exams, and match into your dream residency program. This question and all others are free for 48 hours; after that, it’s available to Premium subscribers only.
A 31-year-old female presents to the emergency department at 28 weeks’ gestation because of left-sided calf swelling, tenderness, and dull pain. Upon examination, the left calf was found to have an increased circumference of three inches compared to the right calf and demonstrated warmth and a bluish discoloration. Her vital signs are all within normal limits. The results of the Homans sign test were positive on the left side. The patient denies chest pain, dizziness, or dyspnea. A venous ultrasound of the left calf reveals a noncompressible popliteal vein. Based on these findings, she should be treated with medication that works via which of the following mechanisms?
A. Directly inhibiting thrombin
B. Inhibiting hepatic epoxide reductase
C. Catalyzing the conversion of plasminogen to plasmin
D. Binding to antithrombin III
E. Irreversibly inhibiting cyclooxygenase 1
Detailed Breakdown of Answers + Correct Answer Below ⏬
ANSWER + QUESTION BREAKDOWN
The mental model used to answer this question comes from our detailed test-taking skills masterclass. Here’s how to think through this question:
Step 1. Read the last line to determine the actual question: “Based on these findings, she should be treated with medication that works via which of the following mechanisms?”
Step 2: Is this a first-, second-, or third-order question?
Answer: 3rd order. 1st: Make a diagnosis. 2nd: Based on your diagnosis, find the most appropriate treatment. 3rd: Identify the mechanism of action of the most appropriate treatment.
Step 3: Ask yourself: “What’s my diagnosis, how do I manage it pharmacologically, and how does that drug work?”
Step 4. Look at the answer choices and select the option most closely resembling your final thought from “Step 3” above.
GENERAL ANALYSIS
The clinical presentation and findings in this case suggest a DVT in the patient's left leg. The presence of a noncompressible popliteal vein on ultrasound confirms this diagnosis. Anticoagulation therapy is the standard treatment to prevent clot propagation and potential complications such as pulmonary embolism in such cases.
ANSWER CHOICES:
CHOICE A: Directly inhibiting thrombin
Explanation: This mechanism is characteristic of direct thrombin inhibitors like dabigatran. While effective, direct thrombin inhibitors are not the most commonly used initial treatment for DVT.
CHOICE B: Inhibiting hepatic epoxide reductase
Explanation: This describes warfarin’s mechanism of action. Warfarin is often used for long-term anticoagulation but not typically for initial treatment due to delayed onset.
CHOICE C: Catalyzing the conversion of plasminogen to plasmin
Explanation: This describes the action of thrombolytics, which dissolve clots. Thrombolytics are not routinely used for DVT unless there is a life-threatening situation like massive pulmonary embolism.
CHOICE D: Binding to antithrombin III
Explanation: This mechanism describes how heparin and low molecular weight heparins (e.g., enoxaparin) work. They enhance the activity of antithrombin III, inhibiting factors Xa and IIa (thrombin). Heparin or low molecular weight heparins are commonly used for the initial management of DVT due to their rapid onset of action.
CHOICE E: Irreversibly inhibiting cyclooxygenase 1
Explanation: This describes the action of aspirin, which affects platelet function. Aspirin is not used in the primary management of a DVT.
FINAL VERDICT…
The standard initial treatment for deep vein thrombosis involves anticoagulants that act quickly to prevent clot extension and embolization. Heparin and low molecular weight heparins (such as enoxaparin) work by binding to antithrombin III, significantly increasing its inhibitory effect on thrombin and factor Xa. This mechanism provides effective anticoagulation and is suitable for immediate management in a hospital setting.
CORRECT ANSWER: D) Binding to antithrombin III
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