**Core Concept**
The pituitary gland is a master gland that regulates the function of other endocrine glands through the secretion of various hormones. It is a complex neuroendocrine system that is sensitive to changes in its structure and function, particularly in the presence of a pituitary lesion.

**Why the Correct Answer is Right**
In a pituitary lesion, the first hormone secretion to be affected is usually the one that is most sensitive to changes in pituitary function. The hormone that is secreted by the anterior pituitary gland, specifically by the lactotroph cells, is **Prolactin**. Prolactin is secreted in response to the inhibitory action of dopamine from the hypothalamus. However, in the presence of a pituitary lesion, the dopamine inhibitory pathway is disrupted, leading to an increase in prolactin secretion. This is known as **hyperprolactinemia**.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the first hormone secretion to be affected in a pituitary lesion is not growth hormone, which is secreted by the somatotroph cells of the anterior pituitary gland.
* **Option B:** This option is incorrect because the first hormone secretion to be affected in a pituitary lesion is not thyroid-stimulating hormone (TSH), which is secreted by the thyrotroph cells of the anterior pituitary gland.
* **Option D:** This option is incorrect because the first hormone secretion to be affected in a pituitary lesion is not adrenocorticotropic hormone (ACTH), which is secreted by the corticotroph cells of the anterior pituitary gland.

**Clinical Pearl / High-Yield Fact**
In a pituitary lesion, the disruption of the dopamine inhibitory pathway can lead to an increase in prolactin secretion, which can have significant clinical implications, including galactorrhea, amenorrhea, and infertility in women, and impotence and gynecomastia in men.

**Correct Answer:** A. Prolactin