Which of the following Plasmodium species has the longest exoerythrocytic cycle and is responsible for relapses occurring up to 3 years after the initial infection?

Explanation

## Exoerythrocytic Cycle Duration and Relapses in Malaria Parasites **Key Point:** *Plasmodium vivax* is responsible for relapses occurring up to 3 years (and sometimes longer) after the initial infection, owing to its dormant hypnozoite stage in the liver. It also has the longest clinically relevant exoerythrocytic cycle when hypnozoite reactivation is considered. ### Comparison of Plasmodium Species by Exoerythrocytic Cycle and Relapse Pattern | Species | Pre-erythrocytic (liver) cycle | Hypnozoites? | Relapse Pattern | | --- | --- | --- | --- | | **P. vivax** | 8–10 days (primary); hypnozoites reactivate months–years later | **Yes** | Relapses up to **3 years** (tropical strains) or longer (temperate strains) | | P. ovale | 9–16 days | Yes | Relapses within 6–12 months | | P. malariae | 16–36 days | **No** | Recrudescence (persistent low-level parasitaemia) over decades — NOT true relapse | | P. falciparum | 9–10 days | No | No true relapse; recrudescence only | **High-Yield (Park's Textbook of Preventive & Social Medicine):** *P. vivax* and *P. ovale* form **hypnozoites** — dormant liver-stage parasites that can reactivate weeks to years later, causing true relapses. *P. vivax* (tropical strains) is classically associated with relapses up to **3 years** after the primary attack. *P. malariae* does NOT form hypnozoites; its long-term persistence is due to chronic low-grade erythrocytic parasitaemia (recrudescence), not relapse. **Clinical Pearl:** A patient returning from a malaria-endemic area who develops fever 1–3 years later should be suspected of *P. vivax* relapse. Radical cure with **primaquine** (after ruling out G6PD deficiency) is required to eliminate hypnozoites and prevent relapse. **Why the original answer (P. malariae) was incorrect:** Although *P. malariae* has the longest *primary* pre-erythrocytic cycle (16–36 days), it does NOT produce hypnozoites and therefore does NOT cause true relapses. Its decades-long persistence is recrudescence from residual blood-stage parasites — a fundamentally different mechanism. The stem specifically asks about "relapses up to 3 years," which is the hallmark of *P. vivax*. **Mnemonic:** **"VO" for relapses** — *Vivax* and *Ovale* have hypnozoites (true relapse); *Falciparum* and *Malariae* do not (recrudescence only). *Reference: Park's Textbook of Preventive & Social Medicine, 26th ed.; Harrison's Principles of Internal Medicine, 21st ed.*