## **Core Concept**
The question revolves around the diagnosis of deep vein thrombosis (DVT) in a postoperative patient. DVT is a condition where a blood clot forms in the deep veins, commonly in the legs. The clinical presentation includes leg pain, swelling, redness, and tenderness. Early diagnosis is crucial to prevent complications such as pulmonary embolism.

## **Why the Correct Answer is Right**
The initial test to establish the diagnosis of DVT often involves non-invasive imaging techniques due to their ease of use and quick turnaround time. **Complete Duplex Ultrasound (Option C)**, which includes both B-mode ultrasound and Doppler ultrasound, is widely regarded as the first-line imaging test for DVT. It can directly visualize the thrombus and assess blood flow. This method is sensitive and specific for detecting DVT, particularly in the proximal veins of the legs.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While a chest X-ray can provide some information on lung status and may show signs suggestive of pulmonary embolism (if it occurs), it is not used for the initial diagnosis of DVT.
- **Option B:** MRI Venography can be used for diagnosing DVT, especially in cases where ultrasound is inconclusive or not feasible. However, it is not typically the initial test due to higher costs, less availability, and longer examination time.
- **Option D:** CT Venography can also diagnose DVT and is particularly useful in detecting thrombosis in the pelvic and abdominal veins. However, it involves radiation and contrast, making it less suitable as an initial test compared to duplex ultrasound.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is the **Wells' Score**, a clinical prediction rule used to estimate the probability of DVT. It includes parameters such as clinical signs and symptoms of DVT, recent surgery or immobilization, and history of DVT/PE. However, regardless of the Wells' score, any high-risk patient warrants immediate investigation. Duplex ultrasound is often the go-to initial imaging due to its accuracy, availability, and non-invasive nature.

## **Correct Answer:** C. Complete Duplex Ultrasound.