## Why Dopamine agonist (cabergoline) is right The structure marked **A** is the optic chiasm, which lies immediately above the pituitary gland. A 2.5 cm macroadenoma (>1 cm) compressing the chiasm from below causes bitemporal hemianopia, with superior temporal quadrant deficits occurring earliest because inferior nasal retinal fibers (from the upper visual field) cross first in the chiasm. The markedly elevated prolactin (850 ng/mL) confirms a prolactinoma, which is the most common functional pituitary tumor. According to Harrison 21e Ch 380, prolactinomas are the first-line treatment with dopamine agonists (cabergoline or bromocriptine), which suppress prolactin secretion and often cause tumor shrinkage, relieving chiasmal compression and visual symptoms. Surgery is reserved for non-functional macroadenomas with mass effect or when medical therapy fails. ## Why each distractor is wrong - **Transsphenoidal surgical resection**: While surgery is indicated for non-functional macroadenomas with visual loss or for acromegaly and Cushing disease, prolactinomas respond excellently to medical therapy with dopamine agonists first. Surgery is not first-line for prolactinomas unless there is apoplexy or medical therapy failure. - **Radiation therapy**: Radiation is reserved for tumors resistant to medical and surgical management, or for aggressive non-functional adenomas. It is not first-line for any pituitary adenoma subtype and carries risk of hypopituitarism. - **Observation with serial imaging**: A macroadenoma causing bitemporal hemianopia with visual field defects represents active mass effect and visual compromise. Observation without intervention would risk permanent vision loss and is inappropriate. **High-Yield:** Prolactinoma = dopamine agonist first-line; non-functional macroadenoma with visual loss = surgery; acromegaly/Cushing = surgery + adjuvant. [cite: Harrison 21e Ch 380]
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