**Core Concept:**
Radiotherapy and chemotherapy are both modalities used in cancer treatment, which can sometimes lead to side effects due to their toxic effects on normal tissues and cells. In this scenario, the patient has presented with features after starting chemotherapy, indicating that the drug causing the side effects is likely from the chemotherapy regimen.

**Why the Correct Answer is Right:**
The correct answer is **D.** Leucopenia, one of the side effects of a chemotherapy regimen, is a decrease in the number of white blood cells in the blood, which increases the risk of infections. Chemotherapy drugs often target rapidly dividing cells, including those of the bone marrow, which produces white blood cells.

**Why Each Wrong Option is Incorrect:**

**A.** Leucopenia is not specifically caused by radiotherapy, but rather chemotherapy drugs.

**B.** Radiotherapy primarily affects rapidly dividing cells in the tumor, while chemotherapy affects both tumor cells and normal cells.

**C.** Radiotherapy does not cause a decrease in white blood cells, which is the hallmark of leucopenia.

**Clinical Pearl:**

When treating cancer patients with chemotherapy, clinicians must carefully monitor for side effects like leucopenia and ensure appropriate management, including regular blood tests and infection prophylaxis.

**Correct Answer:** D. Leucopenia (White Blood Cell Count Reduction)

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In this scenario, the patient presented with features after starting chemotherapy, indicating that the drug causing the side effects is likely from the chemotherapy regimen. Leucopenia, a decrease in the number of white blood cells in the blood, increases the risk of infections. Chemotherapy drugs often target rapidly dividing cells, including those of the bone marrow, which produces white blood cells. Therefore, the correct answer is D, as chemotherapy drugs cause leucopenia. The other options are incorrect because:

A. Leucopenia is not specifically caused by radiotherapy, but rather chemotherapy drugs.
B. Radiotherapy primarily affects rapidly dividing cells in the tumor, while chemotherapy affects both tumor cells and normal cells.
C. Radiotherapy does not cause a decrease in white blood cells, which is the hallmark of leucopenia.