WBR0302

Author	[[PageAuthor::William J Gibson (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry, MainCategory::Genetics
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 3-week-old female is brought to the emergency room after experiencing a seizure. The child was born without abnormalities, in an elective cesarean section. The father reports that the child is lethargic, feeds poorly, and exhibits tremors and jerking movements one hour following feeding. You note that the patient has a strong musty odor and that her skin and hair are particularly light. Which of the following amino acids will most likely be administered to the patient?]]
Answer A	AnswerA::Arginine
Answer A Explanation	AnswerAExp::Arginine is a nonessential amino acid, which serves as a precursor for nitric oxide synthesis.
Answer B	AnswerB::Cysteine
Answer B Explanation	[[AnswerBExp::Folic acid and cysteine should be supplemented in homocystinuria.]]
Answer C	AnswerC::Phenylalanine
Answer C Explanation	AnswerCExp::Phenylalanine supplementation would be disastrous for a patient with phenylketonuria. The deleterious effects of phenylketonuria are caused by the accumulation of phenylalanine, leading to toxic concentrations in the body.
Answer D	AnswerD::Tryptophan
Answer D Explanation	[[AnswerDExp::Tryptophan is required for the synthesis of niacin (vitamin B3). Tryptophan can be deficient in patients with Hartnup disease, malabsorption syndromes, or carcinoid syndrome (resulting from the excessive conversion of tryptophan into serotonin).]]
Answer E	AnswerE::Tyrosine
Answer E Explanation	AnswerEExp::Tyrosine supplementation is essential in patients with phenylketonuria.
Right Answer	RightAnswer::E
Explanation	[[Explanation::The child in this scenario is likely suffering from phenylketonuria. Phenylketonuria (PKU) is an autosomal recessive metabolic genetic disorder, characterized by a loss-of-function mutation in the gene responsible for the production of the hepatic enzyme, phenylalanine hydroxylase. Phenylalanine hydroxylase is necessary to metabolize the phenylalanine into the tyrosine. Untreated PKU can lead to mental retardation and seizures, as well as other serious medical problems. Classic PKU patients are frequently treated with a phenylalanine-restricted diet, supplemented by a medical formula containing amino acids and other nutrients. Educational Objective: Tyrosine becomes essential in patients with phenylketonuria. References: First Aid 2014 page 111]]
Approved	Approved::Yes
Keyword	WBRKeyword::Biochemistry, WBRKeyword::Phenylketonuria, WBRKeyword::Genetics, WBRKeyword::Metabolism, WBRKeyword::Tyrosine, WBRKeyword::Amino acid, WBRKeyword::Amino acid metabolism
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::