Review Question - QID 109849

A 17-year-old girl is brought to her primary care physician by her parents. The parents state that their daughter is unruly and never listens. They think her hygiene has recently declined and the only friends she has are imaginary. The daughter is quiet and seems to have a depressed and withdrawn mood. The patient has a medical history of asthma that is treated with albuterol. Her temperature is 99.5°F (37.5°C), blood pressure is 90/48 mmHg, pulse is 80/min, respirations are 17/min, and oxygen saturation is 98% on room air. The patient's body mass index is 14.5 kg/m^2. Physical exam reveals a sad young woman. Cardiopulmonary exam is within normal limits. Abdominal exam reveals decreased bowel sounds and is non-tender in all quadrants. Dermatologic exam is notable for dry skin. Ophthalmologic exam is notable for the finding in Figure A. Lab values are ordered as seen below.

Hemoglobin: 11 g/dL
Hematocrit: 33%
Leukocyte count: 6,500/mm^3 with normal differential
Platelet count: 197,000/mm^3

Serum:
Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 4.0 mEq/L
HCO3-: 26 mEq/L
BUN: 20 mg/dL
Glucose: 62 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 9.0 mg/dL
AST: 12 U/L
ALT: 10 U/L

Which of the following is most likely deficient in this patient?