**Core Concept**
Dopamine has an inhibitory effect on prolactin secretion through its action on D2 receptors in the pituitary gland. This inhibitory effect is crucial for regulating prolactin levels in the body. Dopamine also has other functions, including its role in the nigrostriatal pathway, where it acts on D2 receptors to regulate movement and coordination.

**Why the Correct Answer is Right**
Blocking D2 receptors with antipsychotic medications can lead to an increase in prolactin levels, causing symptoms such as galactorrhea (spontaneous milk production) and amenorrhea (absence of menstruation). Excessive lactation, **Option D**, is a direct consequence of elevated prolactin levels. Gonadal dysfunction, **Option B**, can also occur due to hyperprolactinemia, leading to decreased libido and fertility issues. Headache, **Option C**, is a common side effect of antipsychotic medications, but it is not directly related to the blockade of D2 receptors in the pituitary gland. Visual disturbances, **Option A**, are more likely related to the blockade of D2 receptors in the retina or the dopaminergic pathways in the visual cortex, rather than the pituitary gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Visual disturbances are not a direct consequence of blocking D2 receptors in the pituitary gland. Instead, they may be related to other dopaminergic pathways in the brain.
**Option B:** Gonadal dysfunction is a possible consequence of hyperprolactinemia caused by blocking D2 receptors in the pituitary gland.
**Option C:** Headache is a common side effect of antipsychotic medications, but it is not directly related to the blockade of D2 receptors in the pituitary gland.

**Clinical Pearl / High-Yield Fact**
Blocking D2 receptors in the pituitary gland can lead to hyperprolactinemia, causing symptoms such as galactorrhea and amenorrhea. This is a critical consideration when prescribing antipsychotic medications, especially in patients with a history of reproductive issues.

**✓ Correct Answer: A. Visual disturbance**