Which condition is characterized by a woman having less than 15% of her ideal body weight?

Anorexia nervosa is characterized by an intense fear of gaining weight, a distorted body image, and restricted food intake leading to significantly low body weight. Specifically, a woman with anorexia nervosa may have less than 15% of her ideal body weight, which is a defining criterion for this disorder. The condition can also involve excessive physical activity and extreme dieting behaviors.

In contrast, bulimia nervosa typically involves episodes of binge eating followed by compensatory behaviors, such as purging, but individuals may have a normal weight or may even be overweight. Obesity is defined as a condition of excess body fat and would not relate to body weight being significantly under ideal levels. Orthorexia, although it involves an unhealthy fixation on healthy eating, does not specifically correlate with a defined percentage of body weight compared to ideal weight. Thus, the significant underweight condition linked to anorexia nervosa makes it the correct answer.