**Core Concept:** Hair loss, also known as alopecia, is a common side effect of chemotherapy. Chemotherapy drugs affect rapidly dividing cells, including those in hair follicles, leading to hair loss.

**Why the Correct Answer is Right:** Chemotherapy works by targeting rapidly dividing cells, as these cells are more sensitive to the drugs' effects. In the context of chemotherapy, the drugs aim to kill cancer cells but also affect the normal cells in the body, including those in hair follicles. As hair follicles are rapidly dividing structures, they are susceptible to the cytotoxic effects of chemotherapy drugs.

**Why Each Wrong Option is Incorrect:**

A. **Drugs affecting hair growth:** This option suggests that the drugs specifically target hair growth, which is not the case. Chemotherapy drugs affect rapidly dividing cells, including hair follicles.
B. **Immune system:** While the immune system plays a role in cancer treatment, this option is incorrect because the primary mechanism of action for chemotherapy drugs is not related to the immune system.
C. **Hormones:** Chemotherapy does not primarily affect hormones; it targets rapidly dividing cells, including those affected by hormones. The side effects are the result of the drugs' cytotoxic effects on cells.
D. **Cancer progression:** This option is incorrect because chemotherapy aims to treat cancer by targeting rapidly dividing cancer cells, not by directly affecting cancer progression.

**Clinical Pearl:** Chemotherapy-induced alopecia is a common and often temporary side effect of cancer treatment. Patients undergoing chemotherapy should be informed about this potential side effect and provided with resources to cope with hair loss, such as wigs or hairpieces.

**Correct Answer:** D. Chemotherapy affects rapidly dividing cells, including those in hair follicles, leading to hair loss.