P. vivax morphological identification — DTM&H MCQ

Correct answer: D — Plasmodium vivax

The correct answer is D, Plasmodium vivax. Tertian fever (48 hour cycles) with enlarged, distorted erythrocytes containing prominent Schuffner's dots, amoeboid trophozoites, and mature schizonts with 12-16 merozoites is the classic peripheral blood film picture of P. vivax. Schuffner's dots represent stippling of the infected red cell membrane seen only with the vivax/ovale group, and the combination of cell enlargement plus high merozoite count within schizonts specifically points to vivax rather than ovale. P. vivax has a wide geographic range including the Solomon Islands and other parts of Melanesia and Southeast Asia, consistent with the travel history. UK guidance confirms most non-falciparum malaria in returning travellers is due to P. vivax, with distinct morphology from the other species. Why the other options are wrong: C. Plasmodium falciparum: Infects red cells of normal size with no Schuffner's dots, only ring-form trophozoites and rare crescentic gametocytes are usually seen on peripheral film, as mature schizonts sequester in deep capillaries and rarely circulate. E. Plasmodium knowlesi: Causes quotidian (24 hour) fever, resembles P. malariae morphologically with normal-sized cells and no Schuffner's dots, and schizonts contain fewer merozoites arranged without the enlarged, stippled cell background typical of vivax. B. Plasmodium ovale: Infected cells are oval or fimbriated (fringed) rather than simply enlarged, and mature schizonts contain fewer merozoites, typically 6 to 12, not 12 to 16. A. Plasmodium malariae: Causes quartan fever (72 hour cycles), infects normal or small red cells with no Schuffner's dots, and produces band-form trophozoites and rosette-pattern schizonts with 6 to 12 merozoites. Key point: Enlarged red cells with prominent Schuffner's dots plus schizonts of 12-16 merozoites and tertian periodicity are diagnostic of Plasmodium vivax.

Reference: PHE Advisory Committee on Malaria Prevention / UK Malaria Treatment Guidelines Writing Group, 'UK malaria treatment guidelines 2016', Journal of Infection 2016;72(6):635-649 (https://www.sciencedirect.com/science/article/abs/pii/S0163445316000475)