A 7-month-old boy is brought by his parents for a wellness visit. According to the mother, the patient has had poor weight gain despite getting adequate nutrition
via a combination of breastmilk and solid foods. In addition, the patient has difficulty rolling from a prone to supine position
or sitting upright unassisted. Vitals are within normal limits The patient is at the 5th percentile for weight. Physical examination reveals a frail and pale infant. Enzymatic testing is performed and confirms an absence of uridine monophosphate synthase activity. Which of the following findings is likely to be found on further evaluation?