**Core Concept**
5-fluorouracil (5-FU) is a pyrimidine analogue used in chemotherapy for various cancers, including colorectal, breast, and stomach cancers. It acts as a thymidylate synthase inhibitor, disrupting DNA synthesis and cell division.

**Why the Correct Answer is Right**
The most common side effect of 5-FU is hand-foot syndrome (HFS), also known as palmar-plantar erythrodysesthesia (PPE). This condition is characterized by redness, swelling, and peeling of the skin on the palms and soles. The exact mechanism of HFS is not fully understood, but it is thought to be related to the accumulation of 5-FU and its metabolites in the skin, leading to damage to the epidermal cells and blood vessels. The condition is often seen in patients receiving high doses of 5-FU or those with pre-existing skin conditions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Myelosuppression is a common side effect of many chemotherapeutic agents, but it is not the most common side effect of 5-FU.
* **Option B:** Diarrhea can occur with 5-FU, particularly at high doses, but it is not the most common side effect.
* **Option C:** Nausea and vomiting are common side effects of many chemotherapeutic agents, but they are not the most common side effect of 5-FU.

**Clinical Pearl / High-Yield Fact**
To reduce the risk of HFS, patients receiving 5-FU should be advised to avoid exposure to high temperatures, wear protective clothing, and keep their skin moisturized.

**Correct Answer:** B. Diarrhea