Which of the following is not an intermediate or end product of aerobic metabolism?

• A. Water
• B. Carbon dioxide
• C. Lactic acid
• D. Pyruvic acid

Answer: (C)

In aerobic metabolism, glucose is fully oxidized to carbon dioxide and water. After glycolysis, pyruvate enters the mitochondria and is converted to acetyl-CoA, feeding the citric acid cycle, which along with oxidative phosphorylation ultimately yields CO2 and H2O as end products. Water forms at the end of the electron transport chain, and CO2 is released as the carbon atoms are oxidized in the cycle. Pyruvic acid sits at the junction between glycolysis and the TCA cycle, acting as the key intermediate that is further processed under aerobic conditions. Lactic acid, on the other hand, is produced when oxygen is limited and pyruvate is reduced to lactate to regenerate NAD+, a process that occurs during anaerobic metabolism. Since lactate is not produced in normal aerobic metabolism, it is the correct choice.