Muscle cannot make use of glycogen for energy because of deficiency of-
**Core Concept**
Muscle cells, specifically skeletal muscle fibers, have a unique energy metabolism system that relies heavily on fatty acid oxidation and glycolysis for energy production. Unlike liver cells, muscle cells lack the enzyme glucose-6-phosphatase, which is essential for glucose production from glycogen breakdown.
**Why the Correct Answer is Right**
The correct answer is related to the lack of glucose-6-phosphatase in muscle cells. This enzyme is crucial for converting glucose-6-phosphate into glucose, allowing the liver to produce glucose from glycogen breakdown and release it into the bloodstream. In contrast, muscle cells rely on glycolysis to break down glucose for energy, but they cannot convert glucose-6-phosphate into glucose due to the absence of glucose-6-phosphatase. This limits their ability to use glycogen as a primary energy source.
**Why Each Wrong Option is Incorrect**
**Option A:** Glucose transporter 4 (GLUT4) is indeed involved in glucose uptake in muscle cells, but its deficiency would affect glucose uptake rather than glycogen utilization.
**Option B:** Phosphofructokinase-1 (PFK-1) is a key enzyme in the glycolytic pathway, but its deficiency would affect glycolysis rather than glycogen breakdown.
**Option C:** Pyruvate kinase is another enzyme involved in glycolysis, but its deficiency would also affect glycolysis rather than glycogen utilization.
**Clinical Pearl / High-Yield Fact**
Muscle cells rely on glycolysis and fatty acid oxidation for energy production, and their inability to convert glucose-6-phosphate into glucose limits their ability to use glycogen as a primary energy source.
**Correct Answer:** D. Glucose-6-phosphatase.