La d A boy presented with a fever and chills. Rapid test was positive for specific antigen HRP-2. Which of the following species of Plasmodium is the most likely causative agent?

Explanation

## Correct Answer: C. Plasmodium falciparum The **HRP-2 (Histidine-Rich Protein 2) antigen** is a parasite-derived protein that is pathognomonic for *Plasmodium falciparum*. This antigen is produced exclusively by P. falciparum and is the basis for most rapid diagnostic tests (RDTs) used in India, including the WHO-recommended malaria RDTs. The presence of HRP-2 positivity on a rapid test is therefore diagnostic of P. falciparum malaria. In the Indian context, P. falciparum accounts for approximately 50–60% of malaria cases and is the most severe form, causing cerebral malaria, acute kidney injury, and severe anaemia. The NRHM and RNTCP guidelines in India emphasize HRP-2 based RDTs as the first-line diagnostic tool in endemic areas. The clinical presentation of fever and chills is non-specific and occurs in all Plasmodium species, but the HRP-2 antigen detection is the discriminating diagnostic feature that makes P. falciparum the only correct answer. ## Why the other options are wrong **A. Plasmodium ovale** — P. ovale does not produce HRP-2 antigen. It produces other antigens (pLDH, aldolase) that may be detected on some RDTs, but HRP-2 is specific to P. falciparum. P. ovale is rare in India and causes mild tertian fever; it is not associated with HRP-2 positivity. **B. Plasmodium vivax** — P. vivax does not produce HRP-2 antigen. Although it is the most common malaria parasite in India (40–50% of cases), it is detected by pLDH and aldolase antigens on RDTs, not HRP-2. The HRP-2 test is negative in P. vivax infections, making this option incorrect despite its epidemiological prevalence. **D. Plasmodium malariae** — P. malariae (quartan malaria) does not produce HRP-2 antigen and is extremely rare in India. It causes fever every 72 hours (quartan pattern) and is detected by pLDH and aldolase, not HRP-2. The HRP-2 positivity rules out this species entirely. ## High-Yield Facts - **HRP-2 antigen** is produced exclusively by *Plasmodium falciparum* and is the basis for rapid diagnostic tests (RDTs) recommended by WHO and RNTCP in India. - **P. falciparum** accounts for 50–60% of malaria cases in India and is responsible for severe complications: cerebral malaria, acute kidney injury, severe anaemia, and pulmonary oedema. - **RDT detection targets**: P. falciparum = HRP-2 (and pLDH); P. vivax = pLDH and aldolase; P. ovale = pLDH and aldolase; P. malariae = pLDH and aldolase. - **HRP-2 persists** in blood for 2–3 weeks after parasite clearance, so it can remain positive even after successful treatment. - **Indian malaria epidemiology**: P. vivax (40–50%) and P. falciparum (50–60%) are the two major species; P. ovale and P. malariae are rare. ## Mnemonics **HRP-2 = Falciparum Only** **H**RP-2 = **H**azardous (severe) malaria = **F**alciparum. Remember: HRP-2 is the 'signature antigen' of the most dangerous Plasmodium species. **RDT Antigen Memory: FAP** **F**alciparum = HRP-2 (and pLDH); **A**ll others (vivax, ovale, malariae) = pLDH + **A**ldolase. Use this to distinguish P. falciparum from the rest on RDT interpretation. ## NBE Trap NBE pairs HRP-2 with fever/chills (non-specific symptoms) to test whether students know that HRP-2 is a *specific antigen marker* rather than relying on clinical presentation. Students who focus only on symptoms may incorrectly choose P. vivax (most common in India) instead of P. falciparum (HRP-2 positive). ## Clinical Pearl In Indian endemic areas, when a rapid malaria test is positive for HRP-2, it is P. falciparum until proven otherwise. This distinction is critical because P. falciparum requires urgent artemisinin-based combination therapy (ACT) and close monitoring for severe complications, whereas P. vivax can be managed with chloroquine (in chloroquine-sensitive areas) or ACT. Missing this distinction can delay appropriate treatment escalation. _Reference: Jawetz, Melnick & Adelberg's Medical Microbiology (Parasitology section on Plasmodium); RNTCP Guidelines on Malaria Diagnosis and Management; Harrison's Principles of Internal Medicine Ch. 219 (Malaria)_