**Question:** In muscle, phosphorylase b is inactivated by

A. AMP
B. Creatine kinase
C. Inorganic phosphate
D. Adenosine triphosphate (ATP)

**Correct Answer:** D. Adenosine triphosphate (ATP)

**Core Concept:** In muscle, phosphorylase b is a key component of the glycogenolysis pathway, which is responsible for breaking down glycogen into glucose-1-phosphate during periods of high energy demand. Phosphorylase b is inhibited by high concentrations of ATP, which indicates low energy demand.

**Why the Correct Answer is Right:** ATP is the primary source of energy in cells and plays a crucial role in maintaining cellular homeostasis. In high concentrations, ATP binds to and inhibits phosphorylase b, preventing glycogen breakdown and conserving glycogen stores for future use when energy demand is low.

**Why Each Wrong Option is Incorrect:**

A. AMP: Adenosine monophosphate (AMP) is a breakdown product of ATP and does not directly inactivate phosphorylase b.
B. Creatine kinase: This enzyme is involved in the regulation of creatine phosphate levels and energy production in muscle cells, not glycogenolysis.
C. Inorganic phosphate: Inorganic phosphate is a byproduct of ATP hydrolysis and does not directly inactivate phosphorylase b.

**Why the Correct Answer is Right:**

D. Adenosine triphosphate (ATP) directly inactivates phosphorylase b, ensuring that glycogenolysis occurs only when energy demand is high. ATP serves as a negative regulator of glycogenolysis, preventing excessive breakdown of glycogen and conserving energy stores.

**Clinical Pearl:** A proper understanding of glycogenolysis and the role of ATP in regulating phosphorylase b is essential for maintaining cellular energy homeostasis and preventing muscle dysfunction. In clinical scenarios, such as hypoglycemic coma or prolonged starvation, glycogenolysis may become uncontrolled, leading to hyperglycemia and ketoacidosis.