A 32-year-old man from rural India presents with chronic cough and haemoptysis. Chest X-ray shows an infiltrate in the right upper lobe with a cavity. Which is the most common site of pulmonary tuberculosis on chest imaging?

Explanation

## Most Common Site of Pulmonary TB on Chest Imaging ### Anatomical Distribution **Key Point:** The right upper lobe is the most common site of pulmonary tuberculosis, accounting for approximately 40–50% of cases, followed by the left upper lobe (30–35%). ### Why Upper Lobes? The predilection for upper lobes is due to: 1. **Higher oxygen tension** — TB is an obligate aerobe; upper lobes have the highest PaO₂ 2. **Reduced lymphatic drainage** — slower clearance of organisms 3. **Gravity-dependent perfusion** — supine sleeping position favours apical seeding ### Frequency Distribution Table | Site | Frequency | Notes | | --- | --- | --- | | Right upper lobe | 40–50% | **Most common** | | Left upper lobe | 30–35% | Second most common | | Right lower lobe | 5–10% | Rare in primary TB | | Left lower lobe | 5–10% | Rare in primary TB | | Right middle lobe | < 5% | Least common | **High-Yield:** When you see a cavity on CXR in an Indian patient with chronic cough, think **right upper lobe first**. This is a high-frequency NEET PG question stem. ### Clinical Pearl Bilateral upper lobe involvement is seen in reactivation TB; unilateral disease is more common in primary TB. The right upper lobe dominance is consistent across all age groups and TB types (primary, reactivation, endobronchial). ### Why Not Other Sites? - **Left upper lobe:** Second most common, but still less frequent than right upper lobe - **Lower lobes:** Rare in primary TB; seen in immunocompromised states or aspiration TB - **Middle lobe:** Least common; occasionally involved in endobronchial TB