A $5-$year$-$old child presents with developmental delays, a musty odor in urine, and phenylalanine levels significantly elevated in blood tests. Based on this clinical presentation, which of the following statements best describes the underlying biochemical defect and its implications for treatment? Question $5$ options: The child likely has Maple Syrup Urine Disease, which requires a diet low in branched$-$chain amino acids to prevent neurological damage. The child likely has Hartnup disorder, which requires a diet rich in neutral amino acids to improve symptoms. The child likely has Phenylketonuria $($PKU$),$ which requires strict dietary management to limit phenylalanine intake and prevent cognitive impairment. The child likely has Alkaptonuria, which necessitates supplementation with vitamin C to reduce the accumulation of homogentisic acid. The child likely has Homocystinuria, which can be managed with high doses of vitamin B$6$ to enhance the activity of cystathionine synthase.