Pneumocystis carinii is a fungus because -a) rRNA, mitochondrial protein gene sequence & presence of thymidylate synthaseb) Cell wall contains glucansc) Antifungals are effective against P. cariniid) Commonest infection in AIDS
**Core Concept**
Pneumocystis carinii, now referred to as Pneumocystis jirovecii, is a pathogen that has long been debated as to its taxonomic classification. The question revolves around the characteristics that classify it as a fungus.
**Why the Correct Answer is Right**
The correct answer is based on the genetic analysis of Pneumocystis jirovecii, which has revealed that it possesses characteristics similar to those of fungi. The presence of rRNA and mitochondrial protein gene sequences is indicative of fungal origin. Additionally, the presence of thymidylate synthase, an enzyme involved in DNA synthesis, further supports the fungal classification of Pneumocystis jirovecii.
**Why Each Wrong Option is Incorrect**
**Option B:** The cell wall of Pneumocystis jirovecii does not contain glucans, which are a characteristic component of fungal cell walls. This makes option B an incorrect choice.
**Option C:** While antifungal agents, such as trimethoprim-sulfamethoxazole, are effective against Pneumocystis jirovecii, this is not a characteristic that defines its fungal classification. Instead, it is a result of the pathogen's sensitivity to these agents.
**Option D:** Pneumocystis jirovecii is indeed a common opportunistic infection in AIDS patients, but this is a consequence of the pathogen's ability to infect immunocompromised individuals, rather than a characteristic that defines its fungal classification.
**Clinical Pearl / High-Yield Fact**
Pneumocystis jirovecii pneumonia (PCP) is a classic opportunistic infection in HIV/AIDS patients, often presenting with non-specific symptoms such as cough, shortness of breath, and fever. Diagnosis is typically made through the detection of cysts or trophic forms in induced sputum or lung tissue.
**Correct Answer:** A. rRNA, mitochondrial protein gene sequence & presence of thymidylate synthase