Repeated USMLE Questions Step 1 Review-169- Pharmacology

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Q- A patient receiving trimethoprim-sulfamethoxazole develops megaloblastic anemia. Which mechanism explains this adverse effect?

A- Folate metabolism inhibition

B- Hemoglobin oxidation

C- Inhibition of erythropoietin synthesis

D- Vitamin B12 malabsorption

A- Folate metabolism inhibition- Trimethoprim inhibits dihydrofolate reductase, and sulfamethoxazole blocks dihydropteroate synthase, impairing folate metabolism and potentially causing megaloblastic anemia.

 

Q- A patient with hypertension is treated with clonidine. What is the primary mechanism by which this drug lowers blood pressure?

A- Alpha-1 receptor blockade

B- Central alpha-2 receptor stimulation

C- Inhibition of angiotensin II receptors

D- Sodium channel blockade

B- Central alpha-2 receptor stimulation- Clonidine activates α₂ receptors in the brainstem, reducing sympathetic outflow and lowering peripheral vascular resistance and blood pressure.

 

Q- A patient with tuberculosis treated with rifampin develops decreased effectiveness of multiple medications. What pharmacologic property of rifampin causes this effect?

A- CYP450 enzyme inhibition

B- CYP450 enzyme induction

C- Decreased intestinal absorption

D- Renal excretion competition

B- CYP450 enzyme induction- Rifampin strongly induces hepatic cytochrome P450 enzymes, accelerating metabolism of many drugs such as oral contraceptives, warfarin, and antiretrovirals.

 

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Dr Ray Makar
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