She is 20 and has the right to refuse — discharge with outpatient follow-up.

Assess her decisional capacity for this specific decision. Severe malnutrition may impair cognitive function sufficiently to compromise capacity. Additionally, if the patient's refusal places her in imminent medical danger, the situation may meet criteria for involuntary commitment.

Admit her involuntarily — anorexia nervosa always constitutes grave disability.

Defer to the parents since she is only 20.