**Core Concept**
Muscle glycogen stores play a crucial role in providing glucose to the bloodstream, especially during exercise. This process is mediated by the breakdown of glycogen to glucose through the action of specific enzymes.

**Why the Correct Answer is Right**
The correct answer is due to the lack of glucose-6-phosphatase enzyme in muscle cells. Glucose-6-phosphatase is a key enzyme responsible for converting glucose-6-phosphate into glucose, which can then be released into the circulation. In contrast, muscle cells have the enzyme phosphoglucomutase, which converts glucose-6-phosphate into glucose-1-phosphate, but not glucose-6-phosphatase to convert it into free glucose. This limitation restricts the muscle's ability to release glucose into the bloodstream.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the regulation of glucose release from muscle glycogen.

**Option B:** The lack of insulin in the bloodstream does not affect the direct release of glucose from muscle glycogen stores.

**Option C:** The presence of insulin actually inhibits glycogen breakdown in muscle cells, rather than facilitating it.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that muscle glycogen stores are primarily used for energy production within the muscle cells themselves, with minimal contribution to the circulating glucose pool. This is in contrast to the liver, which has both glucose-6-phosphatase and insulin-independent pathways for releasing glucose into the bloodstream.

**Correct Answer: D. Glucose-6-phosphatase**