## Core Concept
Anorexia nervosa and bulimia nervosa are two distinct eating disorders characterized by different patterns of eating behavior, weight management, and psychological factors. The key to differentiating between them lies in understanding their diagnostic criteria, particularly focusing on weight status, eating patterns, and compensatory behaviors.

## Why the Correct Answer is Right
The correct answer, , highlights that a key differentiator between anorexia nervosa and bulimia nervosa is the presence of significantly low body weight in anorexia nervosa. Individuals with anorexia nervosa typically have a body mass index (BMI) below 18.5 due to restrictive eating patterns, whereas those with bulimia nervosa may have a normal or even high BMI, as they often engage in binge eating followed by compensatory behaviors like purging or excessive exercise, without significant weight loss.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because while binge eating can occur in both disorders, it is more characteristic and frequent in bulimia nervosa. However, some individuals with anorexia nervosa can also exhibit binge-eating behaviors, making this not the most reliable differentiator.
- **Option B:** This option is incorrect because self-induced vomiting is a compensatory behavior seen in bulimia nervosa but can also occur in some cases of anorexia nervosa, particularly in the purging subtype. Thus, it does not distinctly differentiate between the two disorders.
- **Option D:** This option is incorrect because fear of gaining weight is a core psychological feature of both anorexia nervosa and bulimia nervosa. This fear contributes to the restrictive eating patterns seen in both disorders, although it manifests differently.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl to remember is that the presence of significantly low body weight is a hallmark of anorexia nervosa, distinguishing it from bulimia nervosa. Clinicians often use BMI as a quick reference to assess the severity of weight issues in eating disorders.

## Correct Answer: D. Significant low body weight.