Final answer:
Tanya seems to exhibit signs of anorexia nervosa, which is an eating disorder characterized by very low food intake, excessive exercise, and a distorted body image.
Step-by-step explanation:
From the observations provided about Tanya's behavior, it appears that Tanya may be suffering from anorexia nervosa. This eating disorder is characterized by individuals eating very little and maintaining body weight that is significantly below average through starvation and/or excessive exercise. Moreover, people with anorexia nervosa often have a distorted body image and may see themselves as overweight even when they are extremely thin. Tanya's relentless exercise, obsession with dieting despite being at a low weight already, and her inability to recognize that she does not need to lose more weight, are classic signs of this disorder.