**Core Concept**
Menorrhagia in a postmenopausal woman with symptoms of lethargy, constipation, cold intolerance, and muscle stiffness suggests a hormonal imbalance. The clinical presentation points towards hypothyroidism, a condition characterized by decreased production of thyroid hormones (T3 and T4).

**Why the Correct Answer is Right**
The symptoms described are classic for primary hypothyroidism, which is caused by a deficiency of thyroid-stimulating hormone (TSH) receptors or the thyroid gland's ability to produce thyroid hormones. The decreased production of thyroid hormones leads to decreased metabolism, resulting in lethargy, constipation, and cold intolerance. The muscle stiffness is likely due to decreased muscle protein synthesis and increased inflammation. The menorrhagia may be related to the hormonal imbalance, particularly the decrease in estrogen levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Polycystic ovary syndrome (PCOS) is a hormonal disorder affecting women of reproductive age, characterized by irregular menstrual cycles, cysts on the ovaries, and insulin resistance. It does not typically present with menorrhagia and the symptoms described.
**Option B:** Cushing's syndrome is a hormonal disorder caused by excess cortisol levels, leading to symptoms such as weight gain, hypertension, and glucose intolerance. It does not typically present with menorrhagia and the symptoms described.
**Option C:** Pheochromocytoma is a rare, usually benign, tumor of the adrenal gland that produces excess catecholamines, leading to symptoms such as hypertension, tachycardia, and sweating. It does not typically present with menorrhagia and the symptoms described.

**Clinical Pearl / High-Yield Fact**
The classic symptoms of hypothyroidism are often remembered using the mnemonic "Lethargy, Cold intolerance, Constipation, Muscle stiffness, and Myxedema" (LCCMM). Myxedema is a symptom of severe hypothyroidism characterized by puffy skin and swelling.

**Correct Answer:** C.