**Core Concept**
Secondary amenorrhea refers to the cessation of menstrual periods in a woman who has previously menstruated. This condition is often associated with hormonal imbalances, anatomical abnormalities, or systemic diseases that disrupt the normal menstrual cycle.

**Why the Correct Answer is Right**
The most common cause of secondary amenorrhea is polycystic ovary syndrome (PCOS). PCOS is a complex endocrine disorder characterized by hyperandrogenism, ovulatory dysfunction, and polycystic ovaries. In PCOS, the hypothalamic-pituitary-ovarian axis is disrupted, leading to anovulation and subsequent amenorrhea. The exact pathophysiology of PCOS involves insulin resistance, androgen excess, and ovarian stroma hyperplasia, resulting in the characteristic polycystic ovaries.

**Why Each Wrong Option is Incorrect**
**Option A:** Ovarian failure is a common cause of secondary amenorrhea, but it is less common than PCOS. Ovarian failure is characterized by a decline in ovarian function, leading to a decrease in estrogen production and subsequent amenorrhea.
**Option B:** Thyroid disorders are a cause of secondary amenorrhea, but they are less common than PCOS. Hypothyroidism can lead to anovulation and amenorrhea due to the decreased production of thyroid hormones, which affect the hypothalamic-pituitary-ovarian axis.
**Option C:** Pregnancy is a common cause of secondary amenorrhea, but it is often a reversible condition. Pregnancy is a physiological state that disrupts the normal menstrual cycle, leading to amenorrhea.
**Option D:** Coeliac disease is a rare cause of secondary amenorrhea. Coeliac disease is an autoimmune disorder that affects the small intestine and can lead to malabsorption of nutrients, including iron and B vitamins, which are essential for normal menstrual function.

**Clinical Pearl / High-Yield Fact**
PCOS is a common endocrine disorder that affects 5-10% of women of reproductive age. It is often associated with insulin resistance, obesity, and metabolic syndrome, making it a significant public health concern.

**Correct Answer:** D. Coeliac disease is not the most common cause of secondary amenorrhea, but it is a rare and important consideration in the differential diagnosis of secondary amenorrhea.