## Core Concept
Anorexia nervosa is a psychological eating disorder characterized by an intense fear of gaining weight and a distorted body image, leading to restricted food intake and excessive weight loss. A significant consequence of anorexia nervosa, particularly in females, is amenorrhea, which is the absence of menstruation. This condition is closely linked to hormonal imbalances caused by severe weight loss and nutritional deficiencies.

## Why the Correct Answer is Right
The correct answer, **Leptin**, is a hormone that plays a crucial role in regulating energy balance and body weight. It is produced by adipose tissue and signals the hypothalamus about the body's energy status. In individuals with anorexia nervosa, severe weight loss leads to a decrease in leptin levels. Leptin is essential for the normal functioning of the hypothalamic-pituitary-gonadal axis. Decreased leptin levels disrupt this axis, leading to reduced production of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH), which in turn causes amenorrhea.

## Why Each Wrong Option is Incorrect
- **Option A: Insulin** - While insulin levels can be affected in anorexia nervosa due to changes in glucose metabolism, insulin itself is not directly responsible for the regulation of the menstrual cycle in the context of this disorder.
- **Option B: Ghrelin** - Ghrelin is a hormone that stimulates appetite and is often elevated in individuals with anorexia nervosa. Although it plays a role in energy balance, its decrease is not directly linked to amenorrhea; in fact, ghrelin levels are typically increased in states of fasting or low energy balance.
- **Option D: Estrogen** - Estrogen levels are indeed affected in anorexia nervosa and contribute to amenorrhea. However, the decrease in estrogen is a result of the disrupted hypothalamic-pituitary-gonadal axis, which is primarily triggered by low leptin levels.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that amenorrhea in anorexia nervosa is not just about low body fat but also about the metabolic and hormonal consequences of severe weight loss. Leptin's role in signaling energy status to the reproductive system is critical. A memorable point is that restoration of menstrual function often requires not just weight gain but also a significant increase in leptin levels, which can take time.

## Correct Answer Line
**Correct Answer: C. Leptin.**