**Question:** Radiotherapy works by

A. Inducing apoptosis in cancer cells
B. Blocking DNA synthesis
C. Altering gene expression
D. Inhibiting cell cycle progression

**Core Concept:** Radiotherapy is a cancer treatment method that utilizes ionizing radiation to damage the DNA of rapidly dividing cells, particularly in cancer cells, leading to cell death or impaired growth.

**Why the Correct Answer is Right:** Radiotherapy works by inducing apoptosis, or programmed cell death, in cancer cells. This selective targeting of rapidly dividing cells, including cancer cells, helps eliminate the tumor while minimizing damage to healthy surrounding tissues.

**Why Each Wrong Option is Incorrect:**

A. Blocking DNA synthesis (Option A) is not the primary mechanism of action for radiotherapy. While radiation can indirectly affect DNA synthesis, the main goal is to induce cell death or inhibit growth.
B. Altering gene expression (Option B) is not the primary mechanism of action for radiotherapy. The primary focus is on inducing apoptosis or inhibiting cell division.
C. Involving alterations in gene expression (Option C) is not the main mechanism of radiotherapy. The primary goal is to damage DNA, leading to cell death or growth inhibition.
D. Inhibiting cell cycle progression (Option D) is a secondary effect of radiotherapy, but the primary mechanism focuses on inducing apoptosis in rapidly dividing cells.

**Clinical Pearl:** Radiotherapy can be combined with chemotherapy or other treatments to enhance their effects and improve cancer control. This approach, known as concurrent therapy, is commonly used in the management of various malignancies.

**Correct Answer:** D. Inhibiting cell cycle progression is a secondary effect of radiotherapy, as radiation damages DNA and triggers apoptosis in rapidly dividing cells. This primary mechanism helps control cancer while minimizing damage to surrounding healthy tissues.