**Core Concept**
The patient's symptoms suggest a systemic disorder involving the endocrine system, specifically affecting the adrenal glands and possibly the pituitary gland. This condition is characterized by an imbalance of hormones that regulate metabolism, growth, and development.

**Why the Correct Answer is Right**
The patient's presentation is consistent with Addison's disease, a rare endocrine disorder caused by the insufficient production of cortisol and aldosterone by the adrenal glands. This can be due to autoimmune destruction of the adrenal glands (primary adrenal insufficiency) or decreased ACTH production by the pituitary gland (secondary adrenal insufficiency). The patient's symptoms, such as fatigue, weakness, weight loss, and hyperpigmentation, are classic signs of corticosteroid deficiency. The hyperpigmentation is caused by increased ACTH levels, which stimulate melanocortin receptors, leading to increased melanin production.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for the patient's hyperpigmentation and proximal muscle weakness, which are characteristic of Addison's disease.

**Option B:** This option is incorrect because it does not explain the patient's hyperpigmentation and does not account for the adrenal insufficiency.

**Option C:** This option is incorrect because it does not explain the patient's hyperpigmentation and does not account for the adrenal insufficiency.

**Option D:** This option is incorrect because it does not explain the patient's hyperpigmentation and does not account for the adrenal insufficiency.

**Clinical Pearl / High-Yield Fact**
Addison's disease is often referred to as the "sick gut, sick skin, sick stomach" syndrome, due to the characteristic symptoms of weight loss, hyperpigmentation, and gastrointestinal disturbances. It is essential to consider Addison's disease in patients with unexplained fatigue, weakness, and weight loss, especially in the context of hyperpigmentation.

**Correct Answer:** C. Addison's disease.