**Core Concept**
Compartment syndrome is a condition where increased pressure within a muscle compartment leads to impaired blood flow, causing tissue ischemia and potentially leading to muscle necrosis. This condition can occur due to various causes, such as trauma, fractures, or prolonged pressure on a limb.

**Why the Correct Answer is Right**
The earliest finding in compartment syndrome is typically a decrease in muscle perfusion, leading to a decrease in intramuscular pressure (IMP). As pressure increases, it can lead to a decrease in the pulse pressure and eventually to a decrease in the capillary refill time. The earliest clinical sign is often pain out of proportion to the injury, which is due to the increase in pressure within the compartment. This pain is often exacerbated by passive stretching of the muscles within the compartment.

**Why Each Wrong Option is Incorrect**
* **Option A:** While pain is a common symptom of compartment syndrome, it is not the earliest finding. Pain out of proportion to the injury is a more specific indicator of compartment syndrome.
* **Option B:** Decreased sensation is not an early sign of compartment syndrome. Sensory changes typically occur later in the course of the disease.
* **Option C:** Decreased pulse is not a reliable early sign of compartment syndrome. Pulse can be maintained even in the presence of significant compartment pressure elevation.
* **Option D:** Decreased capillary refill time is a late sign of compartment syndrome. It indicates severe compromise of perfusion and is often associated with more severe tissue ischemia.

**Clinical Pearl / High-Yield Fact**
The 5 Ps of compartment syndrome are: Pain, Pallor, Pulselessness, Paresthesia, and Paralysis. Remembering these symptoms can help you diagnose compartment syndrome early and prevent long-term damage.

**Correct Answer:** A. Pain