A 9-year-old child in a developing country is brought to a clinic by his parents because he has trouble keeping up with his classmates on the playground. Physical examination is remarkable for pulmonary rales. Chest x-ray shows biventricular dilation of the hea. Deficiency of which of the following vitamins is the most likely cause of this child's condition?

Thiamine deficiency is most frequently encountered in alcoholics and in developing countries. Deficiency of this vitamin can take several forms: dilated cardiomyopathy, polyneuropathy and mamillary body degeneration (Wernicke-Korsakoff syndrome). Ascorbic acid (Vitamin C) deficiency causes scurvy, associated with capillary fragility, bony abnormalities, and poor wound healing. Retinol (Vitamin A) deficiency causes blindness and impaired immune responses. Riboflavin deficiency causes cheilosis, glossitis, and dermatitis. Ref: Bender D.A. (2011). Chapter 44. Micronutrients: Vitamins & Minerals. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds),Harper's Illustrated Biochemistry, 29e.