**Core Concept**
Nelson syndrome is a rare condition characterized by the development of a pituitary adenoma, particularly a cortisol-producing adenoma, in patients who have undergone bilateral adrenalectomy. This syndrome highlights the complex relationship between the hypothalamic-pituitary-adrenal (HPA) axis and the regulation of cortisol production.

**Why the Correct Answer is Right**
Bilateral adrenalectomy is a surgical procedure that involves the removal of both adrenal glands. In patients with Cushing's disease, bilateral adrenalectomy is sometimes performed to treat the excess cortisol production caused by a pituitary adenoma. However, in some cases, the removal of the adrenal glands leads to an increase in the production of adrenocorticotropic hormone (ACTH) by the pituitary gland, resulting in the growth of a cortisol-producing pituitary adenoma, which is known as Nelson syndrome. This is because the removal of the negative feedback from cortisol on the pituitary gland allows the ACTH-producing cells to proliferate.

**Why Each Wrong Option is Incorrect**
**Option A:** BRCA2 mutation is associated with an increased risk of breast and ovarian cancer, but it is not related to Nelson syndrome.

**Option B:** Prophylactic craniospinal irradiation is a treatment for certain types of brain tumors, but it is not associated with the development of Nelson syndrome.

**Option C:** Subarachnoid hemorrhage is a type of stroke caused by bleeding into the space surrounding the brain, but it is not related to Nelson syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential for surgeons and endocrinologists to be aware of the risks of Nelson syndrome in patients who undergo bilateral adrenalectomy for Cushing's disease, as this can lead to the development of a potentially life-threatening pituitary adenoma.

**Correct Answer Line**
✓ Correct Answer: D. Bilateral adrenalectomy