This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': "Kreb's cycle is the second metabolic pathway involved in the catabolism of carbohydrates into energy. First substrate of Kreb's cycle is: \nSelect and explain. \nA. Pyruvate, B. Glycine, C. HQ, D. Lipoprotein", 'role': 'user'}, {'content': 'Answer: A \nExplanation: Pyruvate enters the Krebs cycle (Citric acid cycle or Tricarboxylic acid cycle) following its conversion to acetyl-CoA; the reaction is catalyzed by pyruvate dehydrogenase that links glycolysis to the citric acid cycle and produces one molecule of NADH. Ref: Janson L.W., Tischler M.E. (2012). Chapter 6. Carbohydrate Metabolism. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.', 'role': 'assistant'}]