**Core Concept**
Sheehan's syndrome is a rare but serious complication of postpartum hemorrhage, characterized by hypopituitarism due to necrosis of the pituitary gland.

**Why the Correct Answer is Right**
The patient's symptoms, such as profound lethargy, pallor, muscle weakness, failure of lactation, and amenorrhea, are consistent with hypopituitarism. The abruptio placentae with severe bleeding during delivery likely caused a decrease in blood flow to the pituitary gland, leading to its necrosis. This is followed by a deficiency in the secretion of various pituitary hormones, including prolactin, leading to failure of lactation and amenorrhea.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it refers to a condition that is not directly related to the patient's symptoms and history. Sheehan's syndrome is specifically caused by postpartum hemorrhage, not a congenital condition.
* **Option B:** This option is incorrect because it refers to a condition that affects the adrenal glands, not the pituitary gland. The patient's symptoms are consistent with hypopituitarism, not adrenal insufficiency.
* **Option C:** This option is incorrect because it refers to a condition that affects the hypothalamus, not the pituitary gland. While the hypothalamus and pituitary gland are connected, the patient's symptoms are consistent with a direct effect on the pituitary gland.

**Clinical Pearl / High-Yield Fact**
Sheehan's syndrome is a classic example of a condition that can be prevented by prompt and adequate treatment of postpartum hemorrhage. Early recognition and management of postpartum hemorrhage can prevent the development of Sheehan's syndrome and its associated complications.

**Correct Answer:** C. Pituitary gland necrosis.