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 HRP-2 = Hazardous (severe) malaria = Falciparum. Remember: HRP-2 is the 'signature antigen' of the most dangerous Plasmodium species. RDT Antigen Memory: FAP Falciparum = HRP-2 (and pLDH); All others (vivax, ovale, malariae) = pLDH + Aldolase. 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)_