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
Kreb's cycle is the second metabolic pathway involved in the catabolism of carbohydrates into energy. First substrate of Kreb's cycle is: 
Select and explain. 
A. Pyruvate, B. Glycine, C. HQ, D. Lipoprotein
A: Pyruvate
Answer: A: Pyruvate 
Explanation: 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.