**Core Concept**
The patient's presentation of secondary amenorrhoea and galactorrhoea suggests a hormonal imbalance, specifically an excess of prolactin, which can disrupt normal menstrual cycles and lead to milk production in non-pregnant women.

**Why the Correct Answer is Right**
Prolactinomas, a type of pituitary adenoma, are the most common cause of hyperprolactinemia, which can lead to galactorrhoea and amenorrhoea. The tumor secretes excess prolactin, which stimulates lactation and disrupts the normal feedback mechanisms regulating menstrual cycles. The pituitary gland's secretory function is regulated by the hypothalamus, and an overproduction of prolactin can be caused by a prolactin-secreting tumor or other conditions such as hypothyroidism or certain medications.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not address the patient's specific symptoms of galactorrhoea and amenorrhoea, which are characteristic of hyperprolactinemia.
* **Option B:** This option is incorrect because it is a less common cause of secondary amenorrhoea and does not directly explain the patient's galactorrhoea.
* **Option D:** This option is incorrect because it is not a direct cause of secondary amenorrhoea and galactorrhoea, and the patient's symptoms are more likely related to a hormonal imbalance rather than a structural issue.

**Clinical Pearl / High-Yield Fact**
Prolactinomas are often asymptomatic until they cause significant hormonal imbalances, making early diagnosis and treatment crucial to prevent long-term consequences such as infertility and osteoporosis.

**Correct Answer:** C. Prolactinoma.