Bulimia nervosa is defined primarily by which behavior?

Bulimia nervosa is characterized primarily by a cycle of binge eating followed by compensatory behaviors aimed at preventing weight gain. The hallmark of this disorder involves an individual consuming a large amount of food in a short period of time—this is what constitutes the binge eating. Following the binge, individuals engage in various compensatory actions, such as self-induced vomiting, excessive exercise, or the misuse of laxatives, in an effort to counteract the effects of the binge and to manage their weight. This pattern of behavior can lead to severe physical and psychological complications and is a distinguishing feature that sets bulimia apart from other eating disorders.

The other options do not accurately reflect the core behaviors associated with bulimia nervosa. For instance, while extreme weight loss might occur in some cases, it is not a defining characteristic as it is with anorexia nervosa. Social withdrawal and excessive sleep can be symptoms associated with various mental health issues but are not specific to bulimia nervosa. Thus, the key behavior that defines bulimia is indeed the frequent binge eating followed by compensatory behaviors.