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Methotrexate-co-trimoxazole interaction — GPhC CRA MCQ

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HardMusculoskeletal TherapeuticsMethotrexate-co-trimoxazole interactionGPhC CRA

A patient with stage 4 CKD takes ramipril and spironolactone. Potassium is 5.1 mmol/L before a proposed course of trimethoprim. Which pharmacological effect of trimethoprim most directly compounds this patient’s electrolyte risk?

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Correct answer: E — Epithelial sodium-channel blockade producing an amiloride-like effect

Trimethoprim can act at the distal nephron like the potassium-sparing diuretic amiloride, inhibiting epithelial sodium transport and reducing renal potassium excretion. CKD, an ACE inhibitor, spironolactone and a raised baseline potassium make clinically important hyperkalaemia more likely. The prescription therefore warrants review and a monitoring or alternative-treatment plan, but the question tests the mechanism creating that risk.

Reference: Perindopril 8 mg tablets SmPC, interaction section: https://www.medicines.org.uk/emc/product/11529/smpc