**Core Concept**
Sequestration lung refers to a congenital condition where a portion of lung tissue receives its blood supply from the systemic circulation, rather than the pulmonary arteries. This is due to an abnormal connection between the lung's vascular system and the aorta or its branches.

**Why the Correct Answer is Right**
Angiography is the best diagnostic method for sequestration lung because it allows for direct visualization of the systemic arteries that supply the sequestered lung tissue. This is crucial for identifying the specific artery involved and planning surgical resection. The contrast material used in angiography highlights the anomalous vascular connections, making it an essential tool for diagnosing and managing this condition. In contrast, CT scans and MRI can provide some information about the lung's anatomy and blood supply, but they may not clearly demonstrate the systemic arterial supply characteristic of sequestration lung. A barium swallow is not relevant to diagnosing sequestration lung, as it is used to visualize the esophagus and gastrointestinal tract.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan is less sensitive than angiography for detecting the systemic arterial supply characteristic of sequestration lung.
**Option B:** MRI is not typically used for diagnosing sequestration lung, as it may not provide sufficient detail about the vascular connections.
**Option C:** A barium swallow is irrelevant to diagnosing sequestration lung, as it is used to visualize the esophagus and gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
Sequestration lung is often associated with other congenital anomalies, such as diaphragmatic hernias, and may be diagnosed incidentally on imaging studies performed for unrelated conditions.

**✓ Correct Answer: D. Angiography**