**Core Concept**
Compression ultrasonography (CUS) is the preferred imaging modality for diagnosing deep vein thrombosis (DVT) due to its high sensitivity and specificity. This non-invasive technique uses high-frequency ultrasound waves to visualize the deep veins of the leg and detect thrombi.

**Why the Correct Answer is Right**
CUS is the investigation of choice for DVT because it can accurately detect thrombi in the common femoral and popliteal veins. This is achieved by compressing the vein with the ultrasound probe, and if a thrombus is present, the vein will not compress. The procedure is quick, easy to perform, and does not expose the patient to radiation or contrast agents. CUS is also cost-effective and can be used in conjunction with clinical assessment to rule out pulmonary embolism.

**Why Each Wrong Option is Incorrect**
**Option A:** CT pulmonary angiography (CTPA) is not the investigation of choice for DVT, although it can be used to diagnose pulmonary embolism, which is often associated with DVT. CTPA involves radiation exposure and is more expensive than CUS.
**Option B:** MRI venography can be used to diagnose DVT, particularly in cases where CUS is inconclusive. However, it is not the first-line investigation due to its higher cost and longer examination time compared to CUS.
**Option C:** Venography involves injecting contrast agent into the vein to visualize the thrombus. While it can be used to diagnose DVT, it is an invasive procedure and carries a higher risk of complications compared to CUS.

**Clinical Pearl / High-Yield Fact**
Remember that CUS is the investigation of choice for DVT in the emergency department, as it allows for rapid diagnosis and treatment initiation. In patients with suspected DVT, a negative CUS result can be used to rule out the condition, reducing the need for further investigations.

**Correct Answer: C. Compression ultrasonography (CUS) is the preferred imaging modality for diagnosing deep vein thrombosis (DVT) due to its high sensitivity and specificity. This non-invasive technique uses high-frequency ultrasound waves to visualize the deep veins of the leg and detect thrombi.**