This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Carbohydrates, Subject name: Biochemistry
Caffeine, a methyl xanthine, has been added to a variety of cell types. Which one of the following would be expected in various cell types treated with caffeine and epinephrine? 
Select and explain. 
A. Decreased activity of liver PKA, B. Decreased activity of muscle PKA, C. Increased activity of liver pyruvate kinase, D. Decreased activity of liver glycogen synthase
D: Decreased activity of liver glycogen synthase
Answer: D: Decreased activity of liver glycogen synthase 
Explanation: If the phosphodiesterase that degrades cAMP were inhibited (an effect of caffeine) in the presence of epinephrine, cAMP levels would be elevated. PKA would become more active in the liver and muscle; pyruvate kinase would become less active in the liver; and glycogen synthase activity would be decreased in both the muscle and the liver. Phosphorylase activity would be increased in both the muscle and the liver owing to constant phosphorylation by phosphorylase kinase, which is activated by PKA.