Which Plasmodium species can have exoerythrocytic stages capable of causing relapses months or years after initial infection?

• A. P. falciparum
• B. P. ovale
• C. P. malariae
• D. P. cynomolgi

Answer: (B)

Relapses in malaria happen when dormant forms in the liver, called hypnozoites, reactivate long after the initial blood-stage infection. Only parasites that form these hepatic dormant stages can cause relapses. Among the human malaria species in the options, ovale is the one known to produce hypnozoites in the liver, leading to relapses months or years later. The other listed human parasites do not form hypnozoites: falciparum and malariae lead to ongoing or recrudescent blood-stage infections but not true relapse from liver dormancy. A monkey malaria parasite, cynomolgi, is not a standard cause of relapsing malaria in humans. Hence, ovale best fits the question.