A 4-month-old boy is brought to the office for his first visit since arriving in the United States. The patient was recently adopted, and his adoptive mother says the boy is tremulous compared to her biological children. Over the past week, the boy has also had episodes of upward eye deviation and bilateral arm and leg shaking for approximately 2 minutes at a time. Biological family history is not available. His temperature is 36.7 C (98.1 F), blood pressure is 90/40 mm Hg, pulse is 120/min, and respirations are 30/min. Examination shows a fair-skinned infant with blue eyes and a musty body odor. Which of the following amino acids is most likely essential in this patient?

Question

A 4-month-old boy is brought to the office for his first visit since arriving in the United States.  The patient was recently adopted, and his adoptive mother says the boy is tremulous compared to her biological children.  Over the past week, the boy has also had episodes of upward eye deviation and bilateral arm and leg shaking for approximately 2 minutes at a time.  Biological family history is not available.  His temperature is 36.7 C (98.1 F), blood pressure is 90/40 mm Hg, pulse is 120/min, and respirations are 30/min.  Examination shows a fair-skinned infant with blue eyes and a musty body odor.  Which of the following amino acids is most likely essential in this patient?

Quizplus · Accepted Answer

11ec160b_36b6_49bd_9249_3f0129123715_MD0001_00 This patient's clinical presentation suggests phenylketonuria (PKU). Classic PKU (the most common form) occurs most frequently in persons of Scandinavian descent. Individuals who are homozygous for this autosomal recessive disorder have a severe deficiency of phenylalanine hydroxylase, the enzyme responsible for conversion of phenylalanine to tyrosine. Tyrosine becomes an essential amino acid in these patients as it cannot be synthesized from phenylalanine, although most patients receive adequate amounts in their diet and do not require supplementation. Furthermore, the excess phenylalanine inhibits tyrosinase, which normally results in the downstream production of melanin. The lack of melanin causes the fair complexion seen in this patient. Although development is initially normal, most untreated patients develop intellectual disability by age 6 months. Other classic findings include seizures, eczema, light hair/skin pigmentation, and a "musty," odor. However, cognitive impairment can be lessened by early detection (newborn screening) and restriction of phenylalanine intake. (Choice A) Cystinuria is due to impaired transport of renal cystine (a homodimer of cysteine), which leads to cystine renal stone formation. Clinical manifestations typically include flank pain, hematuria, and possible stone passage in children or adolescents. (Choices B, C, and F) The branched-chain amino acids (leucine, isoleucine, valine) are elevated in maple syrup urine disease, an amino acid disorder marked by deficiency of branched-chain alpha-ketoacid dehydrogenase complex. Buildup of the branched-chain amino acids and their metabolites is toxic, leading to feeding difficulties, seizures, cerebral edema, and a sweet odor of the urine. (Choice D) Phenylalanine levels are significantly elevated in PKU, leading to neurologic defects. Therapy depends on dietary restriction of phenylalanine. Educational objective: Phenylketonuria (PKU) results from an inability to convert phenylalanine to tyrosine by the phenylalanine hydroxylase system, making tyrosine an essential amino acid in these patients. Classic clinical features of untreated PKU include intellectual disability, seizures, light pigmentation, and a "musty" odor. __________ References: Phenylalanine hydroxylase deficiency: diagnosis and management guideline. (http://www.ncbi.nlm.nih.gov/pubmed/24385074)

A 4-Month-Old Boy Is Brought to the Office for His