A 47-year-old woman with diabetes mellitus receives the results of a fasting lipid profile

USMLE Question Break Down Cardiovascular

Welcome to MedSmarter’s USMLE Style Question Break Down of the Week.  For those preparing for the USMLE Step 1, we break down a high-yield cardio question this week.  As always you want to begin with reading the last sentence of the vignette first to understand what the question is asking for.

Question Break Down of the Week:

A 47-year-old woman with diabetes mellitus receives the results of a fasting lipid profile that reveals hypercholesterolemia. To reduce the patient’s mortality risk, her physician recommends lifestyle changes and initiates therapy with atorvastatin. Which of the following mechanisms describes the action of atorvastatin in lowering serum levels of LDL cholesterol?

A) Positive feedback to increase 3-hydroxy-3-methylglutaryl coenzyme A lyase activity
B) Competitive inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase
C) Inactivation of 3-hydroxy-3-methylglutaryl coenzyme A synthase
D) Negative feedback to decrease thiolase activity
E) Noncompetitive inhibition of citrate synthase

The correct answer choice is B. Atorvastatin is a statin and works by inhibiting the activity of HMG-CoA reductase, which is the rate-limiting enzyme involved in cholesterol synthesis. By blocking this enzyme, atorvastatin reduces the synthesis of cholesterol, leading to a decrease in serum levels of LDL cholesterol. Option A, positive feedback to increase 3-hydroxy-3-methylglutaryl coenzyme A lyase activity, is incorrect because HMG-CoA lyase is not the correct enzyme targeted by statins. Option C, inactivation of 3-hydroxy-3-methylglutaryl coenzyme A synthase, is incorrect because HMG-CoA synthase is not the correct enzyme targeted by statins. Option D, negative feedback to decrease thiolase activity, is incorrect because thiolase is not involved in cholesterol synthesis. Option E, noncompetitive inhibition of citrate synthase, is incorrect because citrate synthase is not the enzyme targeted by statins.

Did you think the answer was different?

Did you think that the correct answer choice was other than B?  You can view this video for a deeper discussion of why A, C, D, and  E are not the correct answer choices.

Learn to correctly answer basic science knowledge questions and prepare to take your USMLE Step 1 exam.  The MedSmarter roadmap will make your journey to becoming a practicing physician in the United States as painless as possible.

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