A patient treated for Plasmodium vivax malaria with chloroquine returns after 8 months with a fresh episode of vivax parasitaemia. The relapse occurred because chloroquine fails to act on which stage of the parasite?

• A Hypnozoites in hepatocytes ✓
• B Gametocytes in peripheral blood
• C Schizonts in red blood cells
• D Sporozoites inoculated by the mosquito

Explanation

Chloroquine is a blood schizonticide and clears erythrocytic forms, producing clinical cure. P. vivax and P. ovale persist as dormant hypnozoites in hepatocytes, which reactivate weeks to months later causing relapse. Radical cure therefore requires primaquine, a tissue schizonticide active against hypnozoites. Gametocytes are killed by primaquine to block transmission, not by chloroquine, but they cannot cause relapse in the same patient.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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