This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Endocrinology metabolism, Subject name: Physiology
The first step in alcohol metabolism by the liver is the formation of acetaldehyde from alcohol, a chemical reaction catalyzed by: 
Select and explain. 
A. Cytochrome P450, B. NADPH-cytochrome reductase P450, C. Alcohol oxygenase, D. Alcohol dehydrogenase
D: Alcohol dehydrogenase
Answer: D: Alcohol dehydrogenase 
Explanation: Ans. D. Alcohol dehydrogenaseAlcohol dehydrogenase catalyzes the conversion of alcohol to acetaldehyde, which is then converted to acetate. Acetate is then metabolized by hepatocytes. Cytochrome P450 is a primary component of the oxidative enzyme system involved in the metabolism of drugs. NADPH-cytochrome P450 reductase is an enzyme involved in phase I reactions of drug metabolism. There is no such enzyme as alcohol oxygenase. Glycogen phosphorylase is an enzyme involved in glycogen breakdown, not alcohol metabolism.