**Core Concept**
Galactorrhoea and amenorrhoea are symptoms that suggest hyperprolactinemia, a condition characterized by elevated levels of prolactin in the blood. This can be caused by various factors, including pituitary tumors that secrete prolactin, leading to an imbalance in the body's hormonal regulation.

**Why the Correct Answer is Right**
A prolactinoma, a type of pituitary tumor, secretes excess prolactin, causing the symptoms of galactorrhoea (spontaneous milk production) and amenorrhoea (absence of menstruation). The tumor can compress the surrounding normal pituitary tissue, disrupting the production of other hormones and leading to hormonal imbalances. The symptoms presented by the woman, such as galactorrhoea and amenorrhoea, are consistent with a diagnosis of a prolactinoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Pregnancy is not a cause of galactorrhoea and amenorrhoea, as the latter is actually a symptom that can be used to diagnose pregnancy. Pregnancy hormones, such as human chorionic gonadotropin (hCG), can stimulate prolactin production, but this is not the primary cause of galactorrhoea and amenorrhoea.

**Option C:** Sheehan's syndrome is a condition that occurs in postpartum women due to necrosis of the pituitary gland, leading to hypopituitarism. While it can cause amenorrhoea, it is not typically associated with galactorrhoea.

**Option D:** Metastasis to the pituitary gland from other carcinomas can cause various symptoms, including hormonal imbalances. However, the presentation of galactorrhoea and amenorrhoea is more characteristic of a prolactinoma than metastatic disease.

**Clinical Pearl / High-Yield Fact**
Prolactinomas are the most common type of functioning pituitary tumor, accounting for approximately 40% of all pituitary adenomas. They are often asymptomatic in early stages and may only present with symptoms of hyperprolactinemia, making a high index of suspicion and prompt evaluation essential for diagnosis.

**✓ Correct Answer: B. Pituitary tumor**