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We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Folate antagonists: co-trimoxazole?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

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No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.

Folate antagonists MCQ (B11-BIO1-13)

Q: What is the answer to this Biochemistry question?

The best answer is C, Dihydropteroate synthase by sulfamethoxazole and dihydrofolate reductase by trimethoprim. Sulfamethoxazole is a structural analogue of para-aminobenzoic acid and inhibits dihydropteroate synthase, while trimethoprim inhibits bacterial dihydrofolate reductase with much higher affinity than the human enzyme.

Q: What concept does this question test?

It tests Folate antagonists: co-trimoxazole in Biochemistry. Sequential blockade gives synergy and bactericidal activity. Humans take folate from the diet and lack dihydropteroate synthase.

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Biochemistry · Folate antagonists: co-trimoxazole

Sample item

A patient with a urinary tract infection is given co-trimoxazole. Which pair of bacterial enzymes is blocked sequentially in the folate pathway?

ADihydropteroate synthase by trimethoprim and DNA gyrase by sulfamethoxazoleBDihydrofolate reductase by sulfamethoxazole and thymidylate synthase by trimethoprimCDihydropteroate synthase by sulfamethoxazole and dihydrofolate reductase by trimethoprimDThymidylate synthase by sulfamethoxazole and dihydropteroate synthase by trimethoprim

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

C. Dihydropteroate synthase by sulfamethoxazole and dihydrofolate reductase by trimethoprim

Sulfamethoxazole is a structural analogue of para-aminobenzoic acid and inhibits dihydropteroate synthase, while trimethoprim inhibits bacterial dihydrofolate reductase with much higher affinity than the human enzyme. Sequential blockade gives synergy and bactericidal activity. Humans take folate from the diet and lack dihydropteroate synthase.

Option by option

A. Dihydropteroate synthase by trimethoprim and DNA gyrase by sulfamethoxazole
DNA gyrase is the target of fluoroquinolones, and trimethoprim acts on DHFR.
B. Dihydrofolate reductase by sulfamethoxazole and thymidylate synthase by trimethoprim
The sites are misassigned; sulfonamides act on dihydropteroate synthase, not DHFR.
C. Dihydropteroate synthase by sulfamethoxazole and dihydrofolate reductase by trimethoprim (correct)
Correct. Two consecutive steps are blocked, one by each drug.
D. Thymidylate synthase by sulfamethoxazole and dihydropteroate synthase by trimethoprim
Sulfonamides do not inhibit thymidylate synthase, and trimethoprim does not act on dihydropteroate synthase.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Folate antagonists: co-trimoxazole: practice question

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The answer and the reasoning

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