Co-prescribing methotrexate with trimethoprim can cause a serious interaction. What is the most likely consequence?

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Study for the Foundation Year Pharmacy – Clinical Practice Test. Prepare with detailed questions, step-by-step explanations, and test format insights. Enhance your readiness and confidence!

Multiple Choice

Co-prescribing methotrexate with trimethoprim can cause a serious interaction. What is the most likely consequence?

Explanation:
The interaction hinges on antifolate effects. Methotrexate blocks dihydrofolate reductase, reducing tetrahydrofolate and impairing DNA synthesis in rapidly dividing cells. Trimethoprim also inhibits dihydrofolate reductase and can raise methotrexate exposure by decreasing its renal clearance. Together, they have an additive antifolate effect, which most strongly hits bone marrow, leading to myelosuppression. This can manifest as neutropenia, anemia, and thrombocytopenia, with increased infection risk and bleeding. Hypertension, hyperkalemia, and photosensitivity are not the typical consequences of this interaction.

The interaction hinges on antifolate effects. Methotrexate blocks dihydrofolate reductase, reducing tetrahydrofolate and impairing DNA synthesis in rapidly dividing cells. Trimethoprim also inhibits dihydrofolate reductase and can raise methotrexate exposure by decreasing its renal clearance. Together, they have an additive antifolate effect, which most strongly hits bone marrow, leading to myelosuppression. This can manifest as neutropenia, anemia, and thrombocytopenia, with increased infection risk and bleeding.

Hypertension, hyperkalemia, and photosensitivity are not the typical consequences of this interaction.

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