WBR0984

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Serge Korjian)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 1-year-old boy is evaluated for mental retardation and developmental delay. The mother states that the family returned to USA only one day ago. Abroad, the boy had been diagnosed with a genetic disease, and the family has been advised to follow a special diet to manage the child's condition. The mother cannot recall the name of the supplements and she is concerned about her child's health. Metabolic studies demonstrate acidosis and elevated concentrations of urinary leucine, isoleucine, and valine. The physician explains that the patient needs strict dietary restriction along with vitamin supplementation to improve his clinical condition. Three months later, laboratory testing demonstrates significant reduction in the urinary concentration of leucine, isoleucine, and valine. Supplementation with which of the following vitamins most likely improved this patient's condition?]]
Answer A	AnswerA::Pyridoxine
Answer A Explanation	[[AnswerAExp::Pyridoxine (vitamin B6) is a cofactor for glycogen phosphorylase. It is also required in decarboxylation and transamination reactions. Pyridoxne supplementation is indicated in TB-positive patients receiving isoniazid therapy and sideroblastic anemia,]]
Answer B	AnswerB::Biotin
Answer B Explanation	[[AnswerBExp::Biotin (vitamin B7) is a cofactor for carboxylation enzymes.]]
Answer C	AnswerC::Niacin
Answer C Explanation	[[AnswerCExp::Niacin (vitamin B3) is a constituent of NAD+ and NADP+, both of which are important in redox reactions. Its efficacy has been evaluated in dyslipidemia and has been associated with a reduction in VLDL concentration and an increase in HDL concentration.]]
Answer D	AnswerD::Folic acid
Answer D Explanation	[[AnswerDExp::Folic acid (vitamin B9) supplementation is indicated among pregnant women and patients with megaloblastic anemia due to folic acid deficiency.]]
Answer E	AnswerE::Thiamine
Answer E Explanation	[[AnswerEExp::Supplementation with thiamine (vitamin B1) is associated with clinical improvement among patients with thiamine-responsive maple syrup urine disease. Thiamine increases the activity of alpha-keto acid dehydrogenase.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Maple syrup urine disease (MSUD) is an autosomal recessive genetic disease characterized by the deficiency of alpha-keto acid dehydrogenase. Patients with MSUD often appear normal at birth. However, plasma concentrations of branched-chain amino acids (leucine, isoleucine, and valine) gradually increase, and the disease typically manifests during the 2nd or 3rd day of birth. Manifestations include irritability, poor feeding, convulsions, lethargy, opisthotonus, and a distinctive honey-like/burnt sugar urine odor. Patients may also exhibit stereotyped movements referred to as fencing or bicycling. If left untreated, patients die by the 10th day of birth from ketoacidosis and respiratory failure. The diagnosis of MSUD is confirmed by elevated urinary concentrations of branched-chain amino acid, hydroxyacid, and keto acid. In USA, newborn screening for MSUD is necessary for early treatment and prevention of complications. Management of MSUD involves thiamine supplementation (thiamine is a cofactor for alpha-keto acid dehydrogenase) and dietary restrictions, where patients are advised to avoid foods with branched-chain amino acids. Educational Objective: Maple syrup urine disease (MSUD) is an autosomal recessive genetic disease characterized by the deficiency of alpha-keto acid dehydrogenase. Management of MSUD involves thiamine supplementation (thiamine is a cofactor for alpha-keto acid dehydrogenase) and dietary restrictions, where patients are advised to avoid foods with branched-chain amino acids. References: Strauss KA, Puffenberger EG, Morton DH. Maple Syrup Urine Disease. In: Pagon RA, Adam MP, Ardinger HH, et al., editors. GeneReviews®. Seattle (WA): University of Washington, Seattle; 2006. Available from: http://www.ncbi.nlm.nih.gov/books/NBK1319/ First Aid 2014 page 112]]
Approved	Approved::Yes
Keyword	WBRKeyword::Maple syrup urine disease, WBRKeyword::vitamin B1, WBRKeyword::Thiamine, WBRKeyword::Vitamin, WBRKeyword::Supplementation, WBRKeyword::Management, WBRKeyword::MSUD, WBRKeyword::Enzyme, WBRKeyword::Alpha-keto acid dehydrogenase
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