**Question:** DVT, investigation of choice is -

A. D-dimer
B. Ultrasound (US)
C. CT angiography
D. V/Q scan

**Correct Answer: D. V/Q scan**

**Core Concept:** Diagnosis of deep vein thrombosis (DVT) involves identifying the presence of a blood clot in the deep veins of the lower limbs, specifically the calf veins, thigh veins, and pelvic veins.

**Why the Correct Answer is Right:** V/Q (Venous/Qualitative) scan is a nuclear medicine imaging technique that provides information about both venous and pulmonary perfusion. It helps in diagnosing DVT by assessing the balance between venous and pulmonary blood flow. A V/Q scan measures the uptake of radioactive tracers in the veins and lungs. In a patient with DVT, there is a mismatch between venous and pulmonary perfusion, which is evident on the scan.

**Why Each Wrong Option is Incorrect:**

A. D-dimer: A blood test that measures the presence of fragments of fibrin degradation products, indicating recent clot formation or clot resolution. A negative D-dimer test helps rule out DVT, but a positive result does not confirm the diagnosis.

B. Ultrasound (US): An ultrasound is a non-invasive imaging modality used to evaluate the veins for thrombus, but it is operator-dependent, and its sensitivity and specificity are lower compared to V/Q scan or MRI venography.

C. CT angiography: A computed tomography angiography (CTA) is an imaging technique that visualizes the veins and the pulmonary arteries. However, CTA is an ionizing radiation-based imaging technique, which carries a small radiation risk.

**Clinical Pearl:** In clinical practice, a combination of D-dimer test and compression ultrasound is often used as a first-line investigation for suspected DVT. However, when suspicion is high, V/Q scan or MRI venography is the gold standard investigation.