**Question:** A 30-year-old HIV positive patient presents with fever, dyspnoea and non-productive cough. The patient is cyanosed. His chest X-ray reveals bilateral, symmetrical interstitial infiltrates. The most likely diagnosis is -

A. Pneumocystis jirovecii pneumonia
B. Community-acquired pneumonia
C. Tuberculosis
D. Lung abscess

**Correct Answer:** **A. Pneumocystis jirovecii pneumonia**

**Core Concept:**
Pneumocystis jirovecii pneumonia (PCP) is a severe opportunistic infection that primarily affects immunocompromised patients, such as those with HIV infection. It is characterized by diffuse alveolar damage and inflammation, leading to bilateral, symmetrical interstitial infiltrates on chest X-ray, which are typically symmetrical and involve the bases of the lungs.

**Why the Correct Answer is Right:**
In this case, the patient's HIV positivity, fever, dyspnoea, and bilateral interstitial infiltrates on chest X-ray strongly suggest the diagnosis of PCP. The cyanosis and non-productive cough are also consistent with PCP.

**Why Each Wrong Option is Incorrect:**

**A. Community-acquired pneumonia (CAP):** CAP generally presents with productive cough, unilateral infiltrates, and is more common in immunocompetent individuals.

**B. Tuberculosis (TB):** TB usually presents with productive cough, unilateral infiltrates, and is more common in immunocompromised patients, but is less likely to cause cyanosis and bilateral infiltrates in an HIV-positive patient.

**C. Lung abscess:** Lung abscess presents with focal cavities, unilateral infiltrates, and is more likely to cause productive cough and localized symptoms.

**Clinical Pearl:**
In patients with HIV infection, PCP should be considered in the differential diagnosis for fever, dyspnoea, and bilateral interstitial infiltrates on chest X-ray, even in the absence of productive cough. Timely diagnosis and treatment can prevent severe complications and improve the patient's prognosis.