**Core Concept**
Radiation-induced alopecia is a common side effect of radiation therapy, particularly in the scalp. The development of alopecia is related to the dose and duration of radiation exposure, as well as the sensitivity of hair follicles to ionizing radiation. The scalp is particularly susceptible to radiation-induced alopecia due to its high rate of cell turnover.

**Why the Correct Answer is Right**
Radiation-induced alopecia is a result of damage to the hair follicle stem cells, which are responsible for hair growth. When the scalp receives a dose of 4 Gy, the radiation causes damage to the rapidly dividing cells in the hair follicle, leading to a cessation of hair growth and eventual loss of hair. This process typically occurs within 2-3 weeks after radiation exposure, as the damaged hair follicles are unable to produce new hair.

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no established timeline for alopecia development at a dose of 1 Gy. Lower doses may not be sufficient to cause significant damage to hair follicle stem cells.
* **Option B:** Alopecia development is not typically seen within 4-6 weeks after radiation exposure at a dose of 4 Gy. This timeline is more consistent with the development of radiation-induced fibrosis.
* **Option C:** Alopecia development at a dose of 4 Gy is not typically seen within 1-2 weeks after radiation exposure. This timeline is more consistent with the development of acute radiation syndrome.

**Clinical Pearl / High-Yield Fact**
Radiation-induced alopecia is a dose-dependent effect, with higher doses resulting in more severe and prolonged hair loss. It is essential to take measures to protect the scalp during radiation therapy to minimize the risk of alopecia.

**Correct Answer:** C. 2-3 weeks.