**Core Concept**
Pulmonary embolism (PE) diagnosis involves identifying the presence of a blood clot in the pulmonary arteries. The most reliable method requires direct visualization of the clot's location and extent.

**Why the Correct Answer is Right**
Angiography, specifically pulmonary angiography, involves injecting contrast material into the pulmonary arteries and using X-ray imaging to visualize the clot. This technique provides precise information about the clot's size, location, and relationship to surrounding structures. The high-resolution images enable radiologists to accurately diagnose PE and assess its severity. Pulmonary angiography is considered the gold standard for PE diagnosis due to its high sensitivity and specificity.

**Why Each Wrong Option is Incorrect**
**Option A:** Plain skiagram (chest X-ray) may show indirect signs of PE, such as the Westermark sign or Hampton's hump, but it is not sensitive or specific enough to diagnose PE reliably.
**Option B:** Perfusion (ventilation-perfusion scan) can help identify areas of lung that are not receiving adequate blood flow, suggesting PE. However, it is less accurate than pulmonary angiography and may produce false-negative results.
**Option C:** Electrocardiography (ECG) may show nonspecific changes in patients with PE, such as T-wave inversion or right bundle branch block. However, ECG findings are not reliable enough to diagnose PE definitively.

**Clinical Pearl / High-Yield Fact**
Pulmonary angiography is the most reliable method for diagnosing PE, but it is an invasive procedure and may be associated with complications, such as bleeding or contrast nephropathy. Therefore, it is typically reserved for patients with high clinical suspicion of PE and negative or inconclusive results from non-invasive tests.

**✓ Correct Answer: D. Angiography**