**Core Concept**
Compartment syndrome is a serious condition characterized by increased pressure within a muscle compartment, leading to impaired blood flow and potentially irreversible damage. The pathophysiology involves elevated pressure within the fascial compartment, causing ischemia and ultimately leading to muscle necrosis and nerve damage.

**Why the Correct Answer is Right**
The first sign of compartment syndrome is pain, which is often described as a severe, aching, or burning sensation. This occurs due to the elevated pressure within the muscle compartment, which compresses the nerves and blood vessels. The pain is typically out of proportion to the injury and can be exacerbated by passive stretching of the affected muscles. The pain is often described as "rest pain" or "pain at rest," as it is not relieved by rest or elevation of the affected limb.

**Why Each Wrong Option is Incorrect**
**Option B:** Tingling is a symptom of nerve compression, but it is not the first sign of compartment syndrome. Tingling occurs as a result of prolonged nerve compression, which can lead to nerve damage.
**Option C:** Loss of pulse is a sign of arterial compromise, which can occur in advanced compartment syndrome. However, it is not the first sign of the condition.
**Option D:** Loss of movement is a result of muscle necrosis and nerve damage, which can occur in untreated compartment syndrome. However, it is not the first sign of the condition.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize the first sign of compartment syndrome, which is pain, to prevent long-term damage and disability. The "6 Ps" of compartment syndrome are: Pain, Pallor, Pulselessness, Paralysis, Paresthesia, and Poikilothermia (coldness).

**✓ Correct Answer: A. Pain**