**Core Concept**
Westermark's sign is a radiological finding associated with pulmonary embolism, characterized by a peripheral oligemia or decreased blood flow in the lung periphery due to obstruction of the pulmonary arteries by an embolus. This results in a characteristic appearance on chest radiographs.

**Why the Correct Answer is Right**
In pulmonary embolism, the embolus blocks the flow of blood through the pulmonary arteries, leading to decreased perfusion of the lung periphery. This decreased perfusion is reflected as a peripheral oligemia or decreased vascular markings in the affected areas. Westermark's sign is a classic radiological finding that helps diagnose pulmonary embolism, especially in patients with a high clinical suspicion.

**Why Each Wrong Option is Incorrect**
**Option A:** Pulmonary sequestration is a congenital anomaly characterized by a segment of lung tissue that is not connected to the airways and receives its blood supply from an anomalous systemic artery. While it may show oligemia, it is not associated with Westermark's sign.
**Option B:** Pulmonary alveolar proteinosis is a rare lung disease characterized by the accumulation of surfactant-like protein and phospholipids in the alveoli, leading to impaired gas exchange. It typically shows bilateral ground-glass opacification on chest radiographs, not Westermark's sign.
**Option C:** A.B.P.A (Allergic Broncho-pulmonary Aspergillosis) is a hypersensitivity reaction to Aspergillus antigens, leading to bronchiectasis and eosinophilia. It typically shows centrilobular nodules and bronchiectasis on chest radiographs, not Westermark's sign.

**Clinical Pearl / High-Yield Fact**
Westermark's sign is often seen in conjunction with Hampton's hump, a wedge-shaped infarct in the lung periphery, which is also a classic radiological finding in pulmonary embolism.

✓ Correct Answer: A. Pulmonary embolism