A 32-year-old woman from Odisha presents with fever, chills, and sweating for 3 days. She had returned from a malaria-endemic area 10 days ago. Blood smear shows Plasmodium vivax trophozoites. She is started on chloroquine 600 mg base stat, followed by 300 mg at 6, 24, and 48 hours. On day 5 of treatment, she develops fever again with P. vivax parasites on repeat blood smear. What is the most likely reason for treatment failure?

Explanation

## Clinical Context The patient received appropriate chloroquine dosing for acute P. vivax malaria but developed recurrent fever with parasitemia on day 5—within the window of a relapse rather than a new infection. ## Why Hypnozoite Relapse is Correct **Key Point:** P. vivax and P. ovale form dormant hypnozoites in hepatocytes that are NOT eliminated by chloroquine. Chloroquine kills only the erythrocytic (blood) stage parasites. Relapses occur 2–8 weeks after initial treatment if radical cure (primaquine) is not given. **High-Yield:** In this case, recurrence within 5 days suggests either: 1. Incomplete clearance of erythrocytic parasites (rare with chloroquine-sensitive strains), OR 2. **Relapse from hypnozoites** — the patient received no primaquine for radical cure. **Clinical Pearl:** The standard regimen for P. vivax is: - **Acute phase:** Chloroquine 600 mg base (or equivalent) stat, then 300 mg at 6, 24, and 48 hours - **Radical cure:** Primaquine 0.5 mg/kg/day for 14 days (to eliminate hypnozoites) Without primaquine, relapse is inevitable in P. vivax and P. ovale infections. ## Why Each Distractor is Wrong | Distractor | Reason | |---|---| | Chloroquine-resistant P. vivax | Chloroquine resistance in P. vivax is rare and geographically limited (mainly Indonesia, Papua New Guinea). Odisha is endemic for chloroquine-sensitive P. vivax. | | Inadequate absorption due to vomiting | The question does not mention vomiting; the patient received the full chloroquine course. Malabsorption would cause treatment failure during the acute phase, not a recurrence after initial response. | | Reinfection from a new mosquito bite | Reinfection is possible but less likely than relapse in a patient with untreated hypnozoites. Relapse occurs predictably in the absence of primaquine. | [cite:Harrison 21e Ch 209]