**Core Concept**
Pulmonary embolism (PE) diagnosis involves identifying the presence of a blood clot in the pulmonary arteries, which can be a life-threatening condition. The gold standard for diagnosing PE is a definitive method that provides direct visualization of the clot.

**Why the Correct Answer is Right**
The gold standard for diagnosing pulmonary embolism is **pulmonary angiography**. This involves injecting a contrast agent into the pulmonary arteries through a catheter, allowing for direct visualization of the clot on fluoroscopy. This method is highly sensitive and specific for diagnosing PE, particularly in cases where other imaging modalities, such as computed tomography pulmonary angiography (CTPA), are inconclusive. The contrast agent highlights the clot, making it visible on the X-ray images.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the chest may show indirect signs of PE, but it is not the gold standard for diagnosis.
**Option B:** Ventilation-perfusion scan is a nuclear medicine test that can suggest PE, but it is not definitive.
**Option C:** D-dimer test is a blood test that can indicate the presence of a blood clot, but it is not specific for PE and can be elevated in many other conditions.

**Clinical Pearl / High-Yield Fact**
Pulmonary angiography is an invasive procedure that carries risks, such as bleeding and contrast allergy, so it is typically reserved for cases where the diagnosis is uncertain or the patient's condition is critical.

**Correct Answer: C. Pulmonary angiography.**