**Core Concept:** Anorexia nervosa is a psychiatric disorder characterized by severe fear of weight gain, low body image, and self-imposed underweight condition. Binge-purging type is a subtype in which patients engage in binge eating episodes followed by compensatory behaviors to prevent weight gain.

**Why the Correct Answer is Right:** In the binge-purging type of anorexia nervosa, patients attempt to prevent weight gain by purging, often through vomiting or misuse of laxatives. This aggressive compensatory behavior is aimed at counteracting the potential weight gain from binge eating. Consequently, weight loss and calorie restriction are the primary features of this disorder.

**Why Each Wrong Option is Incorrect:**

A. **Bone loss (osteopenia or osteoporosis)**: Although osteopenia is common in anorexia nervosa, the primary focus is on weight loss and preventing weight gain, not bone loss.

B. **Hypothyroidism**: While hypothyroidism can occur in anorexia nervosa, it is not the least likely change to occur, as the primary focus remains on weight loss and prevention of weight gain.

C. **Dilated pupils**: Dilated pupils are a clinical sign of adrenal insufficiency, not the least likely change in anorexia nervosa. The focus is on weight loss and preventing weight gain.

D. **Hyponatremia**: While hyponatremia can be seen in anorexia nervosa due to excessive water intake and inappropriate antidiuretic hormone (ADH) secretion, it is not the least likely change to occur. The primary focus remains on weight loss and preventing weight gain.

**Clinical Pearl:** In anorexia nervosa, the primary changes are related to weight loss and prevention of weight gain, rather than the listed options. The disorder is characterized by excessive fear of gaining weight, leading to severe calorie restriction and malnutrition.

**Correct Answer:** C. **Dilated pupils**: Dilated pupils are a clinical sign of adrenal insufficiency, which is less likely to occur in anorexia nervosa, as the primary focus is on weight loss and preventing weight gain. The other options are related to specific complications or aspects of the disease process, but do not represent the least likely change in anorexia nervosa.