**Core Concept**
Cortisol is a vital glucocorticoid hormone produced by the adrenal cortex, playing a crucial role in regulating metabolism, immune response, and stress response. Its deficiency can have severe consequences on the body's homeostasis.

**Why the Correct Answer is Right**
Cortisol deficiency leads to a decrease in the production of other vital hormones, such as aldosterone, which regulates electrolyte balance and blood pressure. This deficiency causes a characteristic set of symptoms, including fatigue, weight loss, hypotension, and hyperkalemia. The condition is known as Addison disease, a rare endocrine disorder that can be caused by various factors, including autoimmune destruction of the adrenal glands, genetic mutations, or infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Cushing syndrome is a condition caused by an excess of cortisol, not a deficiency. It is characterized by symptoms such as weight gain, hypertension, and glucose intolerance.
**Option B:** Graves disease is an autoimmune disorder that causes hyperthyroidism, leading to symptoms like weight loss, tremors, and tachycardia. It is unrelated to cortisol deficiency.
**Option D:** Acromegaly is a condition caused by an excess of growth hormone, leading to symptoms such as enlarged hands and feet, joint pain, and sleep apnea. It is not related to cortisol deficiency.

**Clinical Pearl / High-Yield Fact**
Cortisol deficiency can be life-threatening if left untreated, and prompt diagnosis is essential. Clinical presentation may be subtle, but symptoms such as fatigue, weight loss, and hypotension should raise suspicion for Addison disease.

**✓ Correct Answer: C. Addison disease**