A 32-year-old man from rural Chhattisgarh presents with fever, chills, and headache for 4 days. He had returned from a malaria-endemic area 10 days ago. On examination, temperature is 39.5°C, spleen is palpable 2 cm below the costal margin. Blood smear shows P. vivax trophozoites. He is started on chloroquine 600 mg base on day 1, followed by 300 mg base on days 2 and 3. On day 5, he develops fever again with P. vivax parasites on repeat smear. Which of the following is the most likely reason for treatment failure?

Explanation

## Clinical Context The patient has received adequate chloroquine dosing for acute P. vivax malaria (600 mg base day 1, then 300 mg on days 2–3), which is the standard regimen. Despite this, fever recurs with parasitemia on day 5. ## Why Hypnozoite Reactivation Is Correct **Key Point:** P. vivax and P. ovale form dormant liver stages called hypnozoites that are NOT killed by chloroquine or other blood schizonticides. These hypnozoites reactivate weeks to months later, causing relapse. **High-Yield:** Radical cure of P. vivax and P. ovale REQUIRES primaquine (0.5 mg/kg/day for 14 days) or tafenoquine to eliminate hypnozoites. Without primaquine, clinical relapse occurs in 40–50% of patients despite successful acute attack treatment. **Clinical Pearl:** The timing here is crucial — the patient received adequate acute therapy (chloroquine cleared blood parasites initially), but reactivation on day 5 indicates hypnozoite emergence, not treatment failure of the acute attack. ## Mechanism of Relapse ```mermaid flowchart TD A[P. vivax infection]:::outcome --> B[Sporozoites enter liver]:::outcome B --> C[Exoerythrocytic schizogony]:::outcome C --> D[Two pathways]:::decision D -->|Trophozoites| E[Blood stage → Chloroquine kills]:::action D -->|Hypnozoites| F[Dormant in liver]:::outcome E --> G[Acute fever resolves]:::outcome F --> H[Reactivation weeks-months later]:::outcome H --> I[Clinical relapse]:::urgent I --> J[Primaquine needed for radical cure]:::action ``` **Mnemonic:** **CHOP** = **C**hloroquine kills **H**aematozoites (blood stage), **O**nly **P**rimaquine kills hypnozoites.