**Core Concept**
Secondary amenorrhea refers to the cessation of menstruation in a woman who has previously menstruated. It is a significant clinical concern that requires thorough evaluation to identify the underlying cause. The causes of secondary amenorrhea can be broadly categorized into physiological, pathological, and iatrogenic factors.

**Why the Correct Answer is Right**
The most common cause of secondary amenorrhea is pregnancy. When a woman conceives, her body undergoes significant changes to support fetal growth and development. The hormonal changes that occur during pregnancy disrupt the normal menstrual cycle, leading to the cessation of menstruation. The increased levels of human chorionic gonadotropin (hCG) produced by the placenta inhibit the secretion of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn reduces the secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary gland. This results in the atrophy of the ovaries and the cessation of menstruation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Thyroid disorders are a common cause of primary amenorrhea, but they are less likely to cause secondary amenorrhea. Thyroid dysfunction can affect menstrual function, but it is not the most common cause of secondary amenorrhea.
* **Option B:** Polycystic ovary syndrome (PCOS) is a common cause of oligomenorrhea, but it is not the most common cause of secondary amenorrhea. PCOS is characterized by ovulatory dysfunction, insulin resistance, and hyperandrogenism, but it is not typically associated with the complete cessation of menstruation.
* **Option C:** Asherman's syndrome is a rare cause of secondary amenorrhea. It is characterized by the presence of intrauterine adhesions, which can lead to menstrual dysfunction and secondary amenorrhea. However, it is not the most common cause of secondary amenorrhea.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with secondary amenorrhea, it is essential to consider pregnancy as the first and most likely cause. A thorough medical and obstetric history, as well as a physical examination, can help identify pregnancy as the underlying cause.

**Correct Answer:** A. Pregnancy.