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Free notes, MCQs, tables & flowcharts for USMLE Step 3 Biochemistry
An investigator is studying a hereditary defect in the mitochondrial enzyme succinyl-CoA synthetase. In addition to succinate, the reaction catalyzed by this enzyme produces a molecule that is utilized as an energy source for protein translation. This molecule is also required for which of the following conversion reactions?
AOxaloacetate to phosphoenolpyruvate
BPyruvate to acetyl-CoA
CAcetaldehyde to acetate
DGlucose-6-phosphate to 6-phosphogluconolactone
EFructose-6-phosphate to fructose-1,6-bisphosphate
A 2-day-old male infant is brought to the emergency department by ambulance after his parents noticed that he was convulsing and unresponsive. He was born at home and appeared well initially; however, within 24 hours he became increasingly irritable and lethargic. Furthermore, he stopped feeding and began to experience worsening tachypnea. This continued for about 6 hours, at which point his parents noticed the convulsions and called for an ambulance. Laboratories are obtained with the following results: Orotic acid: 9.2 mmol/mol creatinine (normal: 1.4-5.3 mmol/mol creatinine) Ammonia: 135 µmol/L (normal: < 50 µmol/L) Citrulline: 2 µmol/L (normal: 10-45 µmol/L) Which of the following treatments would most likely be beneficial to this patient?
AGalactose avoidance
BAspartame avoidance
CBenzoate administration
DUridine administration
EFructose avoidance
An investigator is studying the effect of extracellular pH changes on the substrates for the citric acid cycle. Which of the following substances is required for the reaction catalyzed by the enzyme marked by the arrow in the overview of the citric acid cycle?
APantothenic acid
BNiacin
CThiamine
DRiboflavin
ELipoic acid
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