Scientists investigating a rare metabolic disorder

A) - Their height has nothing to do with the issue since their weight is adjusted based on their height in the study "the patients were close to the normal weight for their height"
B) - Being the opposite of obesity when diagnosed does not rule out that obesity could be a predisposing factor. Even if this one was true, then you could say being underweight or overweight could be a predisposing factor.
C) - Even if weight loss reduces the symptoms, they already have the disease. The scientists are concerned about a predisposing factor, not reducing symptoms. Irrelevant.
D) - This one is correct. The scientists are looking at the patient's current weight, not what weight they were before being diagnosed, which is not discussed in this article. For example, they could all have been 300 lbs a year ago when diagnosed, but treatment for this disease and side effects could have caused them to all be close to normal weight now.
E) - Irrelevant. The parent's weight is not what is of concern, but the patient's weight. The parent's weight could still be an issue, but that is not what the question is concerned about.