**Question:** Investigation of choice for DVT is-

A. D-dimer
B. Ultrasonography
C. CT Venography
D. Venography

**Correct Answer:** D. Venography

**Core Concept:**

DVT (Deep Vein Thrombosis) is a clinical condition characterized by the formation of a blood clot within a deep vein, predominantly in the legs. The appropriate investigations to diagnose DVT depend on the clinical suspicion and patient's risk factors.

**Why the Correct Answer is Right:**

Venography involves injecting a contrast agent into the vein, which is then imaged using X-rays or CT (Computed Tomography) to visualize the blood clot. Venography is the gold standard for diagnosing DVT, particularly when other non-invasive tests are inconclusive or when a definitive diagnosis is necessary for management decisions.

**Why Each Wrong Option is Incorrect:**

A. D-dimer is a blood test that measures the degradation products of fibrin/fibrinogen, which increases in conditions like DVT, pulmonary embolism, and sepsis. While a raised D-dimer level suggests a possibility of DVT, a negative D-dimer test excludes the diagnosis. Hence, a positive D-dimer test alone is not sufficient for a definitive diagnosis.

B. Ultrasonography is a non-invasive imaging modality that evaluates the blood flow in the veins and valves. It can detect the presence of a blood clot but is limited in its ability to visualize the entire vein system, especially in the pelvis and abdomen.

C. CT Venography involves the use of computed tomography to visualize the veins and detect blood clots. It is particularly useful in cases where venography is not feasible or safe, like when the patient has a history of allergic reactions to iodinated contrast agents, renal dysfunction, or a contraindication to ionizing radiation.

**Clinical Pearl:**

Although D-dimer, ultrasonography, and CT Venography are valuable diagnostic tools for DVT, venography remains the gold standard for detecting and characterizing a blood clot and assessing the extent of the disease. In certain situations, venography might be safer, more accessible, or more accurate than the other options. However, when choosing between D-dimer, ultrasonography, and CT Venography, consider the patient's clinical context, availability, and expertise of the examiner.