Final answer:
An individual with an extreme fear of gaining weight and who is grossly underweight would most likely be diagnosed with anorexia nervosa, an eating disorder involving severe food intake restriction and weight loss.
Step-by-step explanation:
An individual with an extreme fear of gaining weight who is grossly underweight is most likely to be diagnosed with anorexia nervosa.
Anorexia nervosa is an eating disorder characterized by the maintenance of a body weight well below average, influenced by a severe restriction of food intake. Individuals with this condition often exhibit an unrealistic perception of their own low body weight, fearing weight gain and viewing themselves as overweight despite being significantly underweight. This is sometimes associated with body dysmorphic disorder. Anorexia nervosa can result in various severe health issues, including bone loss, organ failure, and in extreme cases, death. The disorder more commonly affects teenagers and young adults and is more prevalent among females. Treatments often include counseling and medical intervention.