A 28-year-old female from coastal Tamil Nadu presents to a government hospital with fever, headache, and myalgia for 4 days. She lives in a fishing hamlet near a lagoon with stagnant water. On blood smear, *Plasmodium vivax* is identified. She is treated with chloroquine and primaquine. On day 3 of treatment, fever resolves and parasitaemia clears. However, she returns 6 weeks later with recurrent fever and parasitaemia. Which epidemiological and parasitological feature best explains this recurrence in the coastal endemic zone?

Explanation

## Clinical Scenario Analysis A 28-year-old female from coastal Tamil Nadu with *P. vivax* malaria was treated with chloroquine + primaquine. Initial response was good (fever resolved, parasitaemia cleared by day 3). Recurrence 6 weeks later raises the question of relapse, reinfection, or inadequate radical cure. ## Why Option C (Inadequate Primaquine Due to G6PD Deficiency Screening Failure) is the Best Answer **Key Point:** The stem states she was treated with chloroquine AND primaquine. Chloroquine clears erythrocytic parasites (confirmed by day 3 clearance). Primaquine is the only drug that eliminates hypnozoites (the liver-stage dormant forms responsible for *P. vivax* relapse). However, primaquine causes **dose-dependent haemolysis in G6PD-deficient individuals**, which is why G6PD screening is mandatory before administration per NVBDCP guidelines. **High-Yield:** In India, G6PD deficiency prevalence is significant (2–15% in endemic coastal populations). If G6PD screening was not performed: - Primaquine may have been withheld or under-dosed to avoid haemolysis - Hypnozoites persist in the liver - Relapse occurs at 3–6 weeks (short-latency Indian *P. vivax* strains) ## Distinguishing the Options | Feature | Option A (Relapse – CQ-resistant strain) | Option B (Reinfection) | **Option C (G6PD screening failure)** | Option D (Recrudescence) | |---|---|---|---|---| | Mechanism | Chloroquine resistance affects erythrocytic stage, NOT hypnozoites | New mosquito bite | Primaquine not given/under-dosed → hypnozoites persist | Incomplete erythrocytic clearance | | Timing | 2–6 weeks | Anytime | 3–6 weeks (short-latency relapse) | Days to weeks | | Consistency with stem | Partially (but CQ-resistance ≠ hypnozoite issue) | Possible but less specific | **Best fits** — explains WHY primaquine failed | Contradicted by day 3 clearance | | Epidemiological specificity | Low | Moderate | **High** — addresses the specific failure point | Low | **Clinical Pearl (Park's Textbook of Preventive & Social Medicine):** NVBDCP mandates G6PD testing before primaquine administration. Failure to screen and appropriately dose primaquine is the **most operationally relevant and epidemiologically specific** explanation for *P. vivax* relapse after apparently complete treatment in Indian endemic zones. ## Why Other Options Are Less Correct - **Option A:** Chloroquine resistance in *P. vivax* affects erythrocytic parasites, not hypnozoites. "Chloroquine-resistant relapse" is a category error — relapse is a hypnozoite phenomenon independent of chloroquine sensitivity. - **Option B:** Reinfection is possible in a perennial transmission zone, but the question asks for the feature that **best explains** recurrence given the treatment context. Reinfection does not account for the specific treatment failure. - **Option D:** Recrudescence is contradicted by the stem — parasitaemia was confirmed cleared on day 3, making incomplete erythrocytic clearance implausible. ## Management Implication Per NVBDCP guidelines, G6PD screening must precede primaquine therapy. In G6PD-deficient patients, weekly low-dose primaquine (0.75 mg/kg/week × 8 weeks) is used instead of the standard daily regimen to achieve radical cure while minimising haemolytic risk. **High-Yield:** *P. vivax* relapse after chloroquine + primaquine in India = suspect G6PD deficiency → inadequate radical cure → hypnozoite persistence.