**Core Concept**
Anorexia nervosa is a complex eating disorder characterized by a distorted body image and restrictive eating habits, leading to significant weight loss and potential organ damage. It involves a combination of psychological, social, and physiological factors.

**Why the Correct Answer is Right**
Anorexia nervosa is often associated with a range of physical and psychological symptoms, including amenorrhea (loss of menstrual cycle), bradycardia (slow heart rate), and osteoporosis (weakened bones). These symptoms occur due to the significant caloric deficit and malnutrition that result from the restrictive eating habits characteristic of anorexia nervosa.

**Why Each Wrong Option is Incorrect**
**Option A:** Amenorrhea is a common symptom in anorexia nervosa due to the significant weight loss and hormonal imbalances that occur as a result of the disorder. Therefore, this option is incorrect because amenorrhea is indeed a common symptom in anorexia nervosa.

**Option B:** Bradycardia is also a common symptom in anorexia nervosa, as the caloric deficit and malnutrition that result from the disorder can lead to a range of cardiovascular complications. This option is incorrect because bradycardia is a known association with anorexia nervosa.

**Option C:** Osteoporosis is a well-documented complication of anorexia nervosa, resulting from the significant weight loss and malnutrition that occur as a result of the disorder. This option is incorrect because osteoporosis is indeed a common symptom in anorexia nervosa.

**Option D:** Weight gain is not a common symptom in anorexia nervosa, as the disorder is characterized by restrictive eating habits and significant weight loss. This option is correct because weight gain is not a typical symptom of anorexia nervosa.

**Clinical Pearl / High-Yield Fact**
It's essential to recognize that anorexia nervosa is a complex disorder that requires a comprehensive treatment approach, involving both psychological and medical interventions. Early recognition and treatment can help prevent long-term complications and improve outcomes.

**Correct Answer: D. Weight gain is not a common symptom in anorexia nervosa, as the disorder is characterized by restrictive eating habits and significant weight loss.**