**Core Concept**
Hypopituitarism is a condition characterized by the underproduction of one or more pituitary hormones, leading to decreased secretion of target gland hormones. The most common presenting feature of adult hypopituitarism is related to the deficiency of sex hormones, which are essential for reproductive and sexual health.

**Why the Correct Answer is Right**
Hypogonadism, or the deficiency of sex hormones (testosterone and estrogen), is the most common presenting feature of adult hypopituitarism. This is because the pituitary gland's primary role is to regulate the secretion of sex hormones by the gonads (testes and ovaries). A deficiency in gonadotropin-releasing hormone (GnRH) or gonadotropins (LH and FSH) from the pituitary gland impairs the production of sex hormones, leading to symptoms such as decreased libido, fatigue, and infertility. The decreased production of sex hormones also affects bone density, leading to osteoporosis and increased risk of fractures.

**Why Each Wrong Option is Incorrect**

**Option A:** Hypothyroidism is not the most common presenting feature of hypopituitarism. While hypothyroidism can occur in hypopituitarism due to decreased production of thyroid-stimulating hormone (TSH), it is not the most common presenting symptom.

**Option C:** Addison's disease, or primary adrenal insufficiency, is not a direct consequence of hypopituitarism. While hypopituitarism can lead to secondary adrenal insufficiency due to decreased production of adrenocorticotropic hormone (ACTH), Addison's disease is a distinct clinical entity.

**Option D:** Hyperprolactinemia is not a common presenting feature of hypopituitarism. In fact, hypopituitarism is more likely to cause decreased production of prolactin due to the deficiency of dopamine, which inhibits prolactin secretion.

**Clinical Pearl / High-Yield Fact**
The clinical presentation of hypopituitarism can be variable and nonspecific, making it challenging to diagnose. A high index of suspicion and careful evaluation of hormone levels are essential to diagnose hypopituitarism.

**✓ Correct Answer: B. Hypogonadism. The most common presenting feature of adult hypopituitarism is hypogonadism, characterized by decreased production of sex hormones.**