**Question:** A 55-year-old man has experienced increasing lethargy for the past 7 months. Physical examination shows the hyperpigmentation of the skin. Vital signs include a temperature of 36.9°C, pulse of 70/min, respirations of 14/min, and blood pressure of 95/65 mm Hg. Laboratory studies include a serum cortisol level of 3 mg/mL at 8:00 A.M. with a serum corticotropin level of 65 pg/mL. Which of the following diseases most often occurs in patients with this disorder?

A. Cushing's syndrome
B. Addison's disease
C. Primary adrenal insufficiency
D. Hyperaldosteronism

**Core Concepts:**

1. **Cushing's syndrome:** It is a clinical condition caused by prolonged exposure to high levels of cortisol, a glucocorticoid hormone, resulting from excessive ACTH production by the pituitary gland or inappropriate cortisol secretion by the adrenal glands.
2. **Addison's disease (primary adrenal insufficiency):** It refers to a deficiency in cortisol and aldosterone production by the adrenal glands, typically due to autoimmune destruction or other causes such as infection or trauma.
3. **Hyperprolactinemia:** It is a condition characterized by abnormally high levels of prolactin in the blood, usually caused by pituitary gland disorders, medication side effects, or hormonal imbalances.
4. **Hyperaldosteronism:** It is an excessive production of aldosterone, a mineralocorticoid hormone, leading to high blood pressure, sodium retention, and potassium loss.

**Why the Correct Answer is B (Addison's disease):**

In this scenario, the patient presents with hyperpigmentation, hypotension, and low blood sugar. The clinical signs are indicative of adrenal insufficiency, primarily affecting the adrenal glands' glucocorticoid (cortisol) and mineralocorticoid (aldosterone) production.

**Why each wrong answer is incorrect:**

A) Cushing's syndrome is caused by excessive ACTH production, not cortisol. The patient's symptoms and laboratory findings are more consistent with adrenal insufficiency.

C) Primary adrenal insufficiency (hypopituitarism) would not typically present with hyperpigmentation, although it may cause hypotension and low blood sugar.

D) Hyperaldosteronism (aldosterone excess) generally results in hypertension, hypokalemia, and hyperkalemic acidosis, not the symptoms and findings described in this scenario.

**Clinical Pearls:**

1. The combination of symptoms and laboratory results in this case is more consistent with Addison's disease (primary adrenal insufficiency), which results in low cortisol and aldosterone production. This leads to hypotension, hyperpigmentation, and hypoglycemia, as seen in the patient described.
2. Differences between Cushing's syndrome, Addison's disease, primary pituitary insufficiency (hypopituitarism