**Core Concept**
Compression ultrasonography (CUS) is a non-invasive diagnostic tool used to evaluate the presence of deep vein thrombosis (DVT) in the lower extremities. It involves the application of pressure to the affected limb using a transducer to compress the veins and assess blood flow.

**Why the Correct Answer is Right**
The correct answer is C. Compression ultrasonography (CUS) is the preferred initial imaging modality for suspected DVT due to its high sensitivity and specificity, ease of use, and non-invasive nature. CUS is particularly effective for detecting proximal DVT, such as in the popliteal and femoral veins. It works by detecting the absence of flow in the affected vein, which is indicative of thrombosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Contrast-enhanced computed tomography (CECT) is not typically used as a first-line imaging modality for DVT due to its higher radiation exposure and lower sensitivity compared to CUS.
**Option B:** Magnetic resonance venography (MRV) is a sensitive imaging modality for DVT but is more expensive and less widely available than CUS, making it a second-line option.
**Option D:** Doppler ultrasound is often used in conjunction with CUS to evaluate blood flow and detect thrombosis but is not typically used as a standalone imaging modality.

**Clinical Pearl / High-Yield Fact**
When diagnosing DVT, it's essential to remember that a negative CUS result does not completely rule out the presence of DVT, especially in patients with distal DVT or those with a high clinical suspicion.

**Correct Answer: C. Compression ultrasonography.**