## **Core Concept**
The patient's symptoms of fever, swelling, and pain in the legs on the 4th post-operative day after a major surgical procedure like right hemicolectomy for carcinoma of the caecum suggest a possible thromboembolic event. This is a common complication in post-operative patients, especially after cancer surgery. The underlying principle here involves the **hypercoagulable state** that often follows surgical procedures, particularly in cancer patients.

## **Why the Correct Answer is Right**
The correct answer, **Deep Vein Thrombosis (DVT)**, is a critical clinical entity to consider in this scenario. DVT can present with symptoms such as fever, swelling, and pain in the affected limb. Post-operative patients, especially those undergoing major surgeries like hemicolectomy for cancer, are at an increased risk of developing DVT due to factors like immobilization, surgical trauma, and the potential hypercoagulable state associated with malignancy. DVT is a significant concern because it can lead to serious complications, including pulmonary embolism.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While superficial thrombophlebitis can cause swelling and pain, it typically does not present with systemic symptoms like fever to the same extent as DVT and is less likely to be a primary concern in a post-operative patient with risk factors for thromboembolic events.
- **Option B:** Lymphedema can cause swelling but usually does not present acutely with fever and pain in the post-operative period following a surgical procedure like hemicolectomy.
- **Option C:** This option is not provided, but based on the context, we focus on why the other provided options are incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Wells' score** can be used to assess the probability of DVT. A score of 2 or more suggests a high probability of DVT. Clinical presentation, patient history, and risk factors should prompt early investigation, often with ultrasound (e.g., Doppler ultrasound), to confirm the diagnosis. Early detection and treatment of DVT are crucial to prevent complications like pulmonary embolism.

## **Correct Answer: D. Deep Vein Thrombosis.**