Kerley B lines on chest X-ray indicate:

• A. Interstitial edema due to fluid in interlobular septa
• B. Pneumothorax
• C. Pleural effusion only
• D. Pneumonia

Answer: (A)

Kerley B lines are small, short horizontal lines seen at the lung bases on a chest X‑ray that reflect thickening of the interlobular septa due to fluid—an interstitial edema pattern. This finding points to fluid accumulation in the lung interstitium, commonly from congestive heart failure or venous congestion, rather than to air in the pleural space or focal lung infection. Pneumothorax would show a separated pleural line with loss of lung markings beyond it; a pleural effusion typically blunts the costophrenic angles and forms a meniscus; pneumonia tends to produce focal air‑space consolidation rather than these septal lines. Hence, Kerley B lines indicate interstitial edema from fluid in the interlobular septa.