A 17-month-old girl presented with regression of her receptive language skills and motor skills such that she could not walk. She continued to regress to the point where she was blind and could no longer swallow. She developed seizures. Her MRI, shown here, demonstrates symmetrical hyperintensity of the periventricular white matter and enhancement of the trigeminal nerve as shown by the arrow. Which of the following diagnostic tests is most likely to reveal the diagnosis?

A. Mutation in the HEXA gene

B. Mutation in the SMPD1 gene

C. Increased lactate/pyruvate ratio

D. Decreased galactocerebrosidase activity

E. Decreased arylsulfatase A enzyme activity