WBR0982
| 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 7 day old newborn baby is noted to have a seizure episode. The baby was normal at birth but was gradually becoming lethargic and feeding poorly since his fifth day of life. Physical examination of the baby reveals hypotonia in all extremities and a distinctive honey like odor of his diapers. Deficiency of which of the following enzymes is most likely causing the manifestations of this baby?]] |
| Answer A | AnswerA::Acyl-co dehydrogenase |
| Answer A Explanation | [[AnswerAExp::Acyl-co dehydrogenase is involved in the fatty acid metabolism. Deficiency in acyl-co A dehydrogenase leads to elevated levels of dicarboxylic acid and decreased levels of glucose and ketones. The baby's symptoms and signs are characteristic of maple syrup disease which is caused by the deficiency in branched-chain alpha ketodehydrogenase.]] |
| Answer B | AnswerB::Lactate dehydrogenase |
| Answer B Explanation | [[AnswerBExp::The baby's symptoms and signs are characteristic of maple syrup disease which is caused by the deficiency in branched-chain alpha ketodehydrogenase and not lactate dehydrogenase.]] |
| Answer C | AnswerC::Branched-chain alpha ketodehydrogenase |
| Answer C Explanation | [[AnswerCExp::Branched-chain alpha ketodehydrogenase is involved in the breakdown of branched amino acids such as leucine, isoleucine and lysine. Deficiency in branched-chain alpha ketodehydrogenase is the underlying cause of maple syrup disease of this baby.]] |
| Answer D | AnswerD::Pyruvate dehydrogenase |
| Answer D Explanation | [[AnswerDExp::Pyruvate dehydrogenase links glycolysis to Krebs cycle. Pyruvate dehydrogenase is associated with the accumulation of pyruvate and alanine as well as with lactic acidosis. The baby's symptoms and signs are characteristic of maple syrup disease which is caused by the deficiency in branched-chain alpha ketodehydrogenase.]] |
| Answer E | AnswerE::Alpha ketoglutarate dehydrogenase |
| Answer E Explanation | [[AnswerEExp::Alpha ketoglutarate dehydrogenase is involved in Krebs cycle. The patient's symptoms and signs are characteristic of maple syrup disease which is caused by the deficiency in branched-chain alpha ketodehydrogenase and not alpha ketoglutarate dehydrogenase.]] |
| Right Answer | RightAnswer::C |
| Explanation | [[Explanation::The newborn baby is noted to have lethargy, poor feeding and neurological manifestations of hypotonia an seizure during the first week of life. The honey-like characteristic smell of the diapers points out to maple syrup urine disease, 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.
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. Reference:
First aid for USMLE step 1, 2013. Page 109. |
| Approved | Approved::No |
| Keyword | WBRKeyword::maple syrup urine disease, WBRKeyword::branched-chain alpha ketodehydrogenase, WBRKeyword::branched-chain |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |