A 3- year-old female child whose growth rate has been in the lower 10th percentile over the last year presents with chronic, nonproductive cough and diarrhea with foul-smelling stools. She is diagnosed as having cystic fibrosis. For which of the following vitamins is this child most likely to be at risk of deficiency?
a) Vitamin C
b) Vitamin B6
c) Folic acid
d) Retinol (vitamin A)
e) Riboflavin (vitamin B2)