The correct option is C Phenylalanine
Alkaptonuria is a genetic disorder which imparts inability to process the amino acids phenylalanine and tyrosine. Mutation in the HGD gene that codes for the enzyme homogentisate 1, 2-dioxygenase is the cause of this disorder. A person homozygous for mutated allele accumulates an intermediate substance called homogentisic acid in the blood and tissues. Thus, the correct answer is option "D".