Pulmonary embolism is diagnosed by:
**Core Concept**
Pulmonary embolism (PE) is a life-threatening condition characterized by the sudden blockage of a pulmonary artery by a blood clot. The diagnosis of PE relies on a combination of clinical presentation, laboratory findings, and imaging studies.
**Why the Correct Answer is Right**
The correct answer is ventilation-perfusion (V/Q) scan, which is a diagnostic imaging test that measures the ratio of ventilation to perfusion in the lungs. In PE, the affected area of the lung is ventilated normally but perfused poorly due to the obstruction of the pulmonary artery. This mismatch is detected by the V/Q scan, making it a useful diagnostic tool for PE. The scan typically shows a "mismatched" pattern, where areas of the lung are ventilated but not perfused.
**Why Each Wrong Option is Incorrect**
**Option A:** Chest X-ray is not a primary diagnostic tool for PE, although it may show indirect signs such as an elevated hemidiaphragm or a pleural effusion.
**Option B:** ECG changes, such as tachycardia or right bundle branch block, can be seen in PE but are not specific or sensitive enough for diagnosis.
**Option C:** D-dimer levels are elevated in PE but are also non-specific and can be elevated in many other conditions, making them a poor standalone diagnostic tool.
**Clinical Pearl / High-Yield Fact**
A classic clinical presentation of PE is the "Well's score", which includes clinical criteria such as tachypnea, tachycardia, and syncope, along with laboratory findings like elevated D-dimer levels. A score of 4 or higher is highly suggestive of PE.
**Correct Answer: D. D-dimer levels are elevated in PE but are also non-specific and can be elevated in many other conditions, making them a poor standalone diagnostic tool.