**Question:** Most common side effect of 5-fluorouracil is:

A. Diarrhoea
B. Nausea and vomiting
C. Neuropathy
D. Peripheral oedema

**Core Concept:**
5-Fluorouracil (5-FU) is a cytotoxic chemotherapy drug used to treat various types of cancer, particularly colorectal, pancreatic, and breast cancer. It is an antimetabolite, which means it acts as a chemical analogue of uracil, inhibiting thymidylate synthase, an enzyme involved in DNA synthesis. The side effects of 5-FU are due to its interference with normal cell division and regeneration processes.

**Why the Correct Answer is Right:**
The correct answer is B (Nausea and vomiting) because these are common gastrointestinal side effects caused by 5-FU. The drug can lead to mucositis (inflammation and ulceration of the gastrointestinal tract), which results in symptoms like nausea and vomiting. In addition, 5-FU can also cause central nervous system effects, leading to these symptoms.

**Why Each Wrong Option is Incorrect:**
A (Diarrhoea) is not the most common side effect. While diarrhoea can occur, it is less common compared to nausea and vomiting.

C (Neuropathy) is incorrect because the most common side effect of 5-FU is gastrointestinal rather than neurological. Neuropathy is a less common side effect, usually caused by higher doses or prolonged therapy.

D (Peripheral oedema) is not the most common side effect. While oedema can occur, it is less common compared to nausea, vomiting, and mucositis.

**Clinical Pearl:**
Understanding the side effects of chemotherapy drugs like 5-FU is crucial for healthcare professionals, particularly oncologists and gastroenterologists, as they help guide treatment decisions, dosages, and supportive care strategies to minimize these adverse events and ensure optimal patient care.