**Core Concept**
Sequestration lung refers to a rare congenital anomaly where a portion of lung tissue receives its blood supply from the systemic circulation, rather than the pulmonary arteries. This typically involves a segment of lung that is not connected to the tracheobronchial tree and has a separate blood supply from the aorta or its branches.

**Why the Correct Answer is Right**
The correct diagnosis of sequestration lung is primarily made by identifying the abnormal blood supply, which can be seen on imaging studies such as CT or MRI scans. The systemic blood supply is often from an intercostal or posterior intercostal artery, or sometimes from the aorta or its branches. This can be visualized using contrast-enhanced imaging, which highlights the anomalous blood vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray may show a mass or consolidation, but it is not specific for diagnosing sequestration lung, and other conditions can also present with similar findings.

**Option B:** Blood gas analysis may show hypoxemia, but this is a non-specific finding that can be seen in many lung conditions, including pneumonia or chronic obstructive pulmonary disease (COPD).

**Option C:** Pulmonary function tests (PFTs) may show a reduction in lung function, but this is also non-specific and can be seen in many other conditions affecting the lungs.

**Clinical Pearl / High-Yield Fact**
A key feature of sequestration lung is the presence of a systemic blood supply, which can be identified on imaging studies. This can be a clue to the diagnosis, especially in cases where the lung tissue appears abnormal or is not connected to the tracheobronchial tree.

**Correct Answer:** D. Angiography