**Core Concept**
Deep vein thrombosis (DVT) is a common postoperative complication, particularly in patients undergoing abdominal surgery, such as right hemicolectomy. This condition occurs due to venous stasis, which results from prolonged periods of immobility during the postoperative period.

**Why the Correct Answer is Right**
The patient's symptoms of fever and pain in the legs on the fourth postoperative day are highly suggestive of DVT. This condition is often caused by the formation of a blood clot in the deep veins of the legs, which can lead to inflammation and pain. The risk of DVT is increased in patients who have undergone major surgery, such as right hemicolectomy, particularly if they have a history of malignancy, as in this case. The clinical entity one should look for in this scenario is DVT, as it is a life-threatening condition that requires prompt diagnosis and treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** Sepsis is a possible complication of postoperative fever, but it is not the most likely explanation for the patient's symptoms of pain in the legs.

**Option B:** Pulmonary embolism is a possible complication of DVT, but it is not the primary clinical entity one should look for in this scenario.

**Option C:** Wound infection is a possible postoperative complication, but it is not typically associated with pain in the legs.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that patients undergoing abdominal surgery have a higher risk of developing DVT, particularly if they have a history of malignancy. A high index of suspicion should be maintained, and appropriate diagnostic tests, such as Doppler ultrasound, should be performed promptly if DVT is suspected.

**Correct Answer:** B.