**Core Concept**
The question is referring to the immediate energy source for sprinters, which involves the breakdown of energy stores in the body. This process is crucial for high-intensity, short-duration activities like sprinting.

**Why the Correct Answer is Right**
During intense exercise like sprinting, the body relies on stored ATP (adenosine triphosphate) for immediate energy. However, the ATP stores in muscles are limited and can be depleted within 8-12 seconds of activity. To replenish these stores, the body relies on the breakdown of phosphocreatine (PCr), which rapidly regenerates ATP. This process is essential for generating the high energy demands required for sprinting.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycogen is an important energy source for endurance activities, but it is not the immediate source for sprinters. Glycogen breakdown is a slower process compared to phosphocreatine breakdown.
**Option B:** Glucose is a primary energy source for the body, but it is not the immediate source for sprinters. Glucose breakdown is a slower process compared to phosphocreatine breakdown.
**Option C:** Fatty acids are an important energy source for endurance activities, but they are not the immediate source for sprinters. Fatty acid breakdown is a slower process compared to phosphocreatine breakdown.

**Clinical Pearl / High-Yield Fact**
For high-intensity, short-duration activities like sprinting, the body relies on stored ATP and phosphocreatine for immediate energy. This is why sprinters can maintain high speeds for short durations before fatigue sets in.

**Correct Answer: D. Phosphocreatine.**