**Core Concept**
Sheehan's syndrome is a rare endocrine disorder that occurs in women who have experienced severe blood loss during childbirth, leading to necrosis of the pituitary gland. This results in a deficiency of one or more anterior pituitary hormones.

**Why the Correct Answer is Right**
Mrs. Rekha's symptoms of amenorrhea, failure of lactation, anemia, lethargy, cold intolerance, and multiple skin infections are indicative of a hormonal imbalance. The severe blood loss during childbirth likely led to necrosis of the pituitary gland, resulting in a deficiency of anterior pituitary hormones. This deficiency can affect various bodily functions, including the reproductive system, metabolism, and immune system.

* **Option A:** Postpartum thyroiditis is incorrect because it typically presents with hyperthyroidism followed by hypothyroidism, and is not associated with severe blood loss during childbirth.
* **Option B:** Sheehan's syndrome is the correct answer, but for completeness, we'll explain why the other options are wrong.
* **Option C:** Pituitary apoplexy is incorrect because it typically presents with sudden onset of severe headache, visual disturbances, and altered consciousness, which are not mentioned in the scenario.
* **Option D:** Adrenal insufficiency is incorrect because it would typically present with symptoms such as hypotension, hyperkalemia, and fatigue, which are not mentioned in the scenario.

**Clinical Pearl / High-Yield Fact**
Sheehan's syndrome can be prevented or minimized with prompt and adequate management of postpartum hemorrhage. Early recognition and treatment of severe blood loss can help preserve pituitary function and prevent long-term hormonal deficiencies.

**Correct Answer:** B. Sheehan's syndrome