**Core Concept**
Deep vein thrombosis (DVT) is a blood clot that forms in the deep veins, often in the legs, which can be a life-threatening condition, especially in post-operative patients. Early diagnosis is crucial to prevent pulmonary embolism and other complications.

**Why the Correct Answer is Right**
The best diagnostic method for DVT is **Compression Ultrasound (CUS)**. CUS is a non-invasive and painless procedure that uses high-frequency sound waves to detect the presence of blood clots in the deep veins. This method is widely accepted as the first-line diagnostic tool for DVT due to its high sensitivity and specificity. The ultrasound waves are compressed between the transducer and the patient's skin, allowing for accurate detection of clot formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Doppler Ultrasound is often used to assess blood flow, but it is not as sensitive as CUS in detecting DVT.
* **Option B:** X-ray is not a suitable method for diagnosing DVT, as it cannot detect soft tissue abnormalities like blood clots.
* **Option D:** CT Angiography is a more sensitive method for detecting DVT, but it exposes the patient to radiation and is more expensive than CUS.

**Clinical Pearl / High-Yield Fact**
In post-operative patients, DVT is a significant concern, especially after major surgeries like hip or knee replacement. A high index of suspicion and prompt diagnosis with CUS can prevent devastating complications like pulmonary embolism.

**Correct Answer: C. Compression Ultrasound (CUS)**