<s>[INST] context: Sho-term regulation of HMG-CoA reductase is achieved by inhibition by phosphorylation (of Serine 872, in humans. Decades ago it was believed that a cascade of enzymes controls the activity of HMG-CoA reductase: an HMG-CoA reductase kinase was thought to inactivate the enzyme, and the kinase, in turn, was held to be activated phosphorylation by HMG-CoA reductase kinase. An excellent review on the regulation of the mevalonate pathway by Nobel Laureates Joseph Goldstein and Michael Brown adds specifics: HMG-CoA reductase is phosphorylated and inactivated by an AMP-activated protein kinase, which also phosphorylates and inactivates acetyl-CoA carboxylase, the rate-limiting enzyme of fatty acid biosynthesis. Thus, both pathways utilizing acetyl-CoA for lipid synthesis are inactivated when energy charge is low in the cell and concentrations of AMP rise. There has been a great deal of research on the identity of upstream kinases that phosphorylate and activate the AMP-activated protein kinase. [/INST] question: Which of the following enzymes is active in phosphorylated form? | options: A: Pyruvate dehydrogenase B: HMG-CoA reductase C: HMG-CoA reductase kinase D: Pyruvate kinase | answer: C </s>