**Core Concept**
The body relies on glycogen as a readily available energy source when glucose levels drop. Liver and muscle glycogen stores play a crucial role in maintaining blood glucose levels during periods of fasting or low glucose intake. The rate of glycogen depletion depends on various factors, including the individual's diet, physical activity level, and hormonal regulation.

**Why the Correct Answer is Right**
Glycogen is broken down into glucose through the action of the enzyme glycogen phosphorylase. In the liver, glycogen is stored in the form of glycogen granules, while in muscle cells, it is stored in the form of glycogen droplets. When glucose levels drop, the hormone glucagon stimulates the breakdown of glycogen in the liver, releasing glucose into the bloodstream. In muscle cells, the hormone adrenaline (epinephrine) stimulates glycogen breakdown to provide energy for muscle contraction. The rate of glycogen depletion is rapid in the initial stages, but it slows down as the stores become depleted. Studies have shown that liver glycogen stores are depleted within 18 hours of fasting, while muscle glycogen stores take longer to deplete.

**Why Each Wrong Option is Incorrect**
**Option A:** 12 hours is too short a time frame for glycogen depletion, as the initial stages of fasting would still have a significant impact on glucose levels.
**Option C:** 24 hours is an overestimation, as glycogen stores would be significantly depleted within this time frame.
**Option D:** 36 hours is too long, as muscle glycogen stores would have been depleted by this time, and the liver would be relying on other energy sources.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that glycogen depletion can lead to hypoglycemia, particularly in individuals with diabetes or those taking certain medications that affect glucose metabolism. Therefore, it's crucial to monitor glucose levels regularly, especially during periods of fasting or low glucose intake.

**✓ Correct Answer: B. 18 hours**