**Core Concept**
Prolactinomas are a type of pituitary adenoma that secrete excess prolactin, leading to hyperprolactinemia. The diagnosis of prolactinoma is often based on the level of prolactin in the blood.

**Why the Correct Answer is Right**
Elevated levels of prolactin are a hallmark of prolactinoma. A prolactin level of ≥ 250 ng/mL (or 250 μg/L) is highly suggestive of a prolactinoma, especially in the absence of other causes of hyperprolactinemia such as pregnancy, hypothyroidism, or liver disease. This is because prolactinomas are typically highly secreting tumors that can cause significant elevations in prolactin levels.

**Why Each Wrong Option is Incorrect**
**Option A:** Prolactin levels < 25 ng/mL are not typically associated with prolactinoma, as they are usually seen in cases of secondary hypopituitarism or primary hypothyroidism. **Option B:** Prolactin levels between 25-99 ng/mL can be seen in a variety of conditions, including pregnancy, stress, and certain medications, making it less specific for prolactinoma. **Option C:** Prolactin levels between 100-249 ng/mL may be seen in some cases of prolactinoma, but they are not as specific as levels ≥ 250 ng/mL, and other causes of hyperprolactinemia should be considered. **Clinical Pearl / High-Yield Fact** A prolactin level of ≥ 250 ng/mL is a red flag for prolactinoma, especially in the absence of other causes of hyperprolactinemia. This is a key point to remember when evaluating patients with hyperprolactinemia. **Correct Answer:** D.