WBR0984

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Biochemistry
Sub Category	SubCategory::General Principles
Prompt	[[Prompt::A 2 year old boy is evaluated for mental retardation and developmental delay. Metabolic studies reveal mild acidosis and elevated levels of leucine, isoleucine and valine. Laboratory tests results following few months of dietary restriction of proteins and vitamin supplementation reveal an improvement in the levels of these amino acids in addition to the absence of acidosis. Supplementation with which of the following vitamins most likely improved the condition of this patient?]]
Answer A	AnswerA::Pyridoxine
Answer A Explanation	[[AnswerAExp::Supplementation with thiamine (vitamin B1) might improve the condition of a thiamine responsive maple syrup disease. Pyridoxine has no role in increasing the activity of branched-chain alpha ketodehydrogenase.]]
Answer B	AnswerB::Biotin
Answer B Explanation	[[AnswerBExp::Supplementation with thiamine (vitamin B1) might improve the condition of a thiamine responsive maple syrup disease. Biotin has no role in increasing the activity of branched-chain alpha ketodehydrogenase.]]
Answer C	AnswerC::Niacin
Answer C Explanation	[[AnswerCExp::Supplementation with thiamine (vitamin B1) might improve the condition of a thiamine responsive maple syrup disease. Niacin has no role in increasing the activity of branched-chain alpha ketodehydrogenase.]]
Answer D	AnswerD::Folic acid
Answer D Explanation	[[AnswerDExp::Supplementation with thiamine (vitamin B1) might improve the condition of a thiamine responsive maple syrup disease. Folic acid has no role in increasing the activity of branched-chain alpha ketodehydrogenase.]]
Answer E	AnswerE::Thiamine
Answer E Explanation	[[AnswerEExp::Supplementation with thiamine (vitamin B1) might improve the condition of a thiamine responsive maple syrup disease. Thiamine has no role in increasing the activity of branched-chain alpha ketodehydrogenase.]]
Right Answer	RightAnswer::E
Explanation	[[Explanation::Maple syrup urine disease is an autosomal recessive disease characterized by the deficiency in branched-chain alpha ketodehydrogenase. Deficiency of this enzyme leads to the elevation of alpha keto acids in the blood due to the impaired breakdown of the branched amino acids isoleucine, leucine and lysine. Maple syrup disease may have a spectrum of clinical presentations that subsequently lead to neurological defects, mental retardation and death if left untreated. There are several types of maple syrup disease: severe neonatal form, mild thiamine non responsive and mild thiamine responsive. In the latter form, thiamine supplementation increases the activity of the enzyme branched-chain alpha ketodehydrogenase. Educational objective: Maple syrup urine disease is an autosomal recessive disease characterized by the deficiency in branched-chain alpha ketodehydrogenase leading to the accumulation of the branched amino acids leucine, isoleucine and lysine in the blood. The mild thiamine responsive form of this disease can be treated by protein restriction and thiamine supplementation. Reference: First aid for USMLE step 1, 2013. Page 109. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Maple syrup disease, WBRKeyword::vitamin B1, WBRKeyword::thiamine
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::