6 month old boy, chronic constipation since the age of 1 week. Current Rx with rectal stimulation, glycerin suppositories, and 4 ounces of prune juice produces string-like stool every 4 days. No vomiting. Growth and development are appropriate for age. Abdominal exam shows distension, no tenderness. Rectal exam, no palpable stool in ampulla. What is next step in mangement?
a. switch to lactose-free formula
b. test of stool for botulism toxin (INCORRECT)
c. stool culture
d. measurement of serum TSH concentration
e. rectal manometry
f. upper GI series