Review Question - QID 108693

A 12-month-old boy is brought in by his parents for a routine healthcare visit. The child was born at 39 weeks gestation in an otherwise uncomplicated pregnancy. Since his last visit, his parents endorse a new skin rash that has been recalcitrant to topical hydrocortisone 1% cream as well as watery, loose stools after meals. His parents deny recent travel or sick contacts but endorse recently weaning the child from breastfeeding to a vegan diet approximately 2 months prior. In addition, his parents endorse poor sleep and decreased activity during the day. Physical exam is notable for the rash shown in Figure A and sparse scalp hair. His temperature is 99.3°F (37.4°C), blood pressure is 92/60 mmHg, pulse is 125/min, respirations are 15/min, and oxygen saturation is 99% on room air. His laboratory results are as follows:

Serum:
Na+: 139 mEq/L
K+: 4.4 mEq/L
HCO3-: 21 mEq/L
Cl-: 100 mEq/L
BUN: 12 mg/dL
Creatinine: 0.8 mg/dL
Glucose: 90 mg/dL

Leukocyte count: 10,000/mm^3
Hemoglobin: 14.4 g/dL
Platelet count: 250,000/mm^3

A deficiency in which of the following substances is the most likely cause of this patient’s disease?