Review Question - QID 210531

A 6-week-old boy is brought to the emergency room with a fever. He was born to a 30-year-old G1P1 mother at 39 weeks gestation via vaginal delivery. The mother underwent all appropriate prenatal care and was discharged from the hospital 1 day after birth. The boy has notable skin erythema around the anus with some serosanguinous fluid. The umbilical stump is present. The patient is discharged from the emergency room with antibiotics. He returns to the emergency room at 32 days of age and his mother reports that he has been clutching his left ear. The left tympanic membrane appears inflamed and swollen. The umbilical stump is still attached and is indurated, erythematous, and swollen. The boy's temperature is 99°F (37.2°C), blood pressure is 100/60 mmHg, pulse is 130/min, and respirations are 20/min. A complete blood count is shown below:

Hemoglobin: 14.0 g/dL
Hematocrit: 42%
Leukocyte count: 16,000/mm^3 with normal differential
Platelet count: 190,000/mm^3

A deficiency in which of the following compounds is most likely the cause of this patient's condition?