A 22-year-old woman is diagnosed with Plasmodium vivax malaria. She is treated with chloroquine for 3 days followed by primaquine for 14 days. Before starting primaquine, the physician orders a specific enzyme assay. Which enzyme is being checked, and why?

• A Pyruvate kinase — deficiency causes haemolytic anaemia with primaquine
• B Dihydrofolate reductase — deficiency reduces primaquine activation
• C CYP2D6 — slow metabolisers fail to activate primaquine
• D Glucose-6-phosphate dehydrogenase (G6PD) — deficiency causes oxidative haemolysis with primaquine ✓

Explanation

Primaquine is an 8-aminoquinoline that generates reactive oxygen species during its metabolism. G6PD-deficient erythrocytes cannot regenerate NADPH to neutralise oxidative stress, leading to acute intravascular haemolysis. G6PD deficiency is the most important contraindication to primaquine, and the test must be performed before starting therapy. Pyruvate kinase deficiency causes chronic haemolytic anaemia but is not specifically triggered by primaquine. CYP2D6 status affects antirelapse efficacy but the urgent pre-treatment screen is for G6PD.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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