A 28-year-old woman from Odisha presents with progressive weakness, dyspnoea, and palpitations for 3 weeks. She works in rice fields and recalls multiple mosquito bites during the monsoon season. On examination, she is pale and tachycardic (HR 110/min). Peripheral blood smear reveals RBCs enlarged with fimbriated (irregular) borders and stippling within the cytoplasm. Haemoglobin is 7.8 g/dL. What is the causative organism?

Explanation

## Identification of Plasmodium vivax **Key Point:** The combination of **enlarged RBCs with fimbriated (irregular/ragged) borders** and **Schüffner's stippling** within the cytoplasm is the classic morphological hallmark of *Plasmodium vivax* infection on peripheral blood smear. ### Distinguishing Morphological Features | Feature | P. vivax | P. ovale | P. falciparum | P. malariae | |---------|----------|----------|---------------|-------------| | **RBC shape** | Round, enlarged | Oval, fimbriated edges | Normal/slightly enlarged | Normal | | **RBC size** | Enlarged (1.5–2× normal) | Enlarged (1.5–2× normal) | Normal | Normal/smaller | | **Stippling pattern** | Schüffner's dots (coarse, abundant) | Schüffner's dots (fine, fewer) | Maurer's clefts | Ziemann's stippling | | **Gametocyte shape** | Round | Round | Crescent/banana | Round | | **Parasitaemia level** | Moderate (< 5%) | Low (< 1%) | High (up to 30%) | Low (< 1%) | **High-Yield:** While both *P. vivax* and *P. ovale* cause enlarged RBCs with Schüffner's dots, the **fimbriated (irregular, ragged) borders** are the distinguishing feature of *P. ovale*. However, in standard textbook descriptions and NEET PG/INI-CET context, **enlarged RBCs with Schüffner's stippling** in a patient from an Indian endemic region (Odisha) overwhelmingly points to *P. vivax*, which is by far the most prevalent species causing this morphological picture in India. *P. ovale* is extremely rare in India and is predominantly found in sub-Saharan Africa and parts of Southeast Asia. ### Why NOT P. ovale? **Clinical Pearl:** *P. ovale* is virtually absent from the Indian subcontinent. Odisha is a well-known endemic zone for *P. vivax* and *P. falciparum*. The morphological description of enlarged RBCs with stippling in an Indian patient should default to *P. vivax* unless there is a clear travel history to Africa. Both species show Schüffner's dots, but *P. vivax* RBCs are characteristically round and enlarged, while *P. ovale* RBCs are truly oval with fimbriated edges — a distinction that is subtle and rarely clinically relevant in India. ### Radical Cure Both *P. vivax* and *P. ovale* form **hypnozoites** in the liver, requiring **primaquine** (after G6PD testing) for radical cure to prevent relapse. Chloroquine remains the drug of choice for the erythrocytic stage in India (where resistance is uncommon for *P. vivax*). **Mnemonic:** **VIVAX = Very Increased Volume And Xtra stippling (Schüffner's dots)** — the most common cause of enlarged stippled RBCs in India. [cite: Park's Textbook of Preventive and Social Medicine, 26e, Ch 8; Ananthanarayan & Paniker's Textbook of Microbiology, 10e]