**Core Concept**
Sheehan's syndrome is a rare but potentially life-threatening condition characterized by postpartum necrosis of the pituitary gland, often resulting from severe blood loss during childbirth. This condition leads to a deficiency of one or more pituitary hormones, including adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH), and prolactin.

**Why the Correct Answer is Right**
To manage Sheehan's syndrome, replacement therapy with synthetic hormones is necessary to alleviate symptoms of hormone deficiency. Hydrocortisone is an essential medication in this context, as it provides exogenous cortisol, a glucocorticoid hormone essential for maintaining blood pressure, immune response, and glucose homeostasis. Hydrocortisone replacement therapy helps to prevent adrenal crisis, a life-threatening complication of untreated cortisol deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Insulin is not a primary treatment for Sheehan's syndrome, although it may be used to manage glucose intolerance associated with cortisol deficiency.

**Option B:** Levothyroxine is used to treat hypothyroidism, which can be a component of Sheehan's syndrome. However, it is not the essential drug in this condition.

**Option C:** Metoclopramide is a prokinetic agent used to manage nausea and vomiting, which can be a symptom of Sheehan's syndrome. However, it is not a treatment for the underlying hormonal deficiency.

**Clinical Pearl / High-Yield Fact**
In patients with Sheehan's syndrome, it is crucial to monitor for signs of adrenal insufficiency, such as dizziness, fatigue, and hypotension, particularly during stress or illness.

**Correct Answer: D. Hydrocortisone.