**Core Concept**
Compression ultrasonography (CUS) is the investigation of choice for diagnosing deep vein thrombosis (DVT). It assesses the compressibility of the veins in the legs and pelvis to identify obstruction caused by thrombi.

**Why the Correct Answer is Right**
CUS is preferred over other imaging modalities due to its high sensitivity and specificity for detecting proximal DVT. It is non-invasive, quick, and can be performed at the bedside. The ultrasound probe is applied over the suspected area, and the operator assesses the venous compressibility. If compression is not achieved, it indicates a thrombus is present. CUS is also useful for detecting acute and chronic DVT, as well as post-thrombotic syndrome.

**Why Each Wrong Option is Incorrect**
**Option A:** CT Pulmonary Angiography (CTPA) is not the investigation of choice for DVT, as it is more expensive and exposes the patient to radiation. While it can detect pulmonary embolism, it is not used for diagnosing DVT.

**Option B:** Magnetic Resonance Venography (MRV) is not typically used for DVT diagnosis due to its high cost, limited availability, and longer examination time. It is more useful for detecting pelvic and abdominal vascular malformations.

**Option C:** Venography involves injecting contrast into the veins to visualize the lumen and detect obstruction. However, it is invasive and carries a risk of contrast allergy, phlebitis, and bleeding.

**Clinical Pearl / High-Yield Fact**
The Wells' score is a clinical prediction rule used to estimate the pre-test probability of DVT. It considers clinical features such as leg swelling, tachycardia, and recent surgery.

**Correct Answer: C. Venography**