**Core Concept:** Compartment syndrome is a medical emergency characterized by increased pressure within a closed anatomical space, leading to reduced blood flow to the tissues and potentially irreversible damage. The key sign is pain with passive stretch of the affected limb.

**Why the Correct Answer is Right:** The earliest reliable sign of compartment syndrome is pain with passive stretch of the affected limb. This is because passive stretch provokes a reflex arc involving the muscle spindle, which is the primary sensory receptor in the limb. This reflex causes an increase in sympathetic activity, leading to vasoconstriction, reduced blood flow, and ultimately tissue ischemia.

**Why Each Wrong Option is Incorrect:**
A. Tenderness: This is a late sign of compartment syndrome, appearing when the compartment pressure reaches a critical level and the pain receptors are activated.
B. Swelling: Swelling does not specifically indicate compartment syndrome but is a nonspecific symptom present in various conditions.
C. Numbness: Numbness is not specific to compartment syndrome and may be seen in other conditions affecting peripheral nerves or the spinal cord.
D. Increased pain with active movement: This sign is not specific to compartment syndrome and can be seen in other painful conditions.

**Clinical Pearl:** The clinical examination should include passive stretch of the affected limb, as pain with passive stretch is the earliest and most specific sign of compartment syndrome. If this sign is present, further evaluation and intervention are essential to prevent tissue damage and long-term disability.

**Correct Answer:** .