## Core Concept
The pituitary stalk, also known as the infundibulum, connects the hypothalamus to the pituitary gland and plays a crucial role in the transport of hypothalamic-releasing and inhibiting hormones to the anterior pituitary gland. Transection of the pituitary stalk disrupts this transport, affecting hormone secretion.

## Why the Correct Answer is Right
Transection of the pituitary stalk leads to a decrease in the delivery of hypothalamic-releasing hormones (e.g., TRH, GnRH, CRH) and inhibiting hormones (e.g., somatostatin, dopamine) to the anterior pituitary gland. This typically results in decreased secretion of anterior pituitary hormones (hypopituitarism), except for prolactin. Prolactin secretion is primarily under tonic inhibitory control by dopamine from the hypothalamus. When the pituitary stalk is transected, the decrease in dopamine delivery to the pituitary gland leads to an increase in prolactin secretion.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but generally, we would expect an increase in prolactin levels due to the reasons mentioned above.
- **Option B:** Similarly, not provided, but one might expect decreased ACTH, TSH, LH, FSH, and GH levels due to reduced stimulation by their respective releasing hormones.
- **Option C:** Without the specific option, it's hard to address directly, but typically, one would expect a decrease in the hormones that are not under inhibitory control or are less directly affected by the loss of hypothalamic input.
- **Option D:** This option is correct and implies that an increase in ACTH (adrenocorticotropic hormone) would not occur as a direct result of pituitary stalk transection. ACTH secretion is primarily regulated by corticotropin-releasing hormone (CRH) from the hypothalamus. Transection would likely decrease CRH delivery, subsequently decreasing ACTH secretion, not increasing it.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that transection of the pituitary stalk leads to **hyperprolactinemia** due to the loss of dopamine's inhibitory effect on prolactin release. This is a critical point for exams and clinical practice, as it helps differentiate the cause of hyperprolactinemia.

## Correct Answer Line
**Correct Answer: D. Increase in ACTH.**