What is the most likely diagnosis for a patient with unilateral, severe, burning pain in a limb lasting months after a fracture, with allodynia and changes in color?

• A. Arterial ulcer
• B. Complex regional pain syndrome
• C. Exertional compartment syndrome
• D. Osteomyelitis

Answer: (B)

This question is about recognizing complex regional pain syndrome (CRPS) after a limb injury. CRPS presents with severe, burning pain that is disproportionately large compared to exam findings and can persist for months after a fracture. It often includes sensory changes such as allodynia (pain from nonpainful stimuli) and autonomic signs like color or temperature changes, edema, and sweating in the affected limb. The underlying mechanism involves peripheral and central sensitization with abnormal sympathetic nervous system activity causing vasomotor instability, hence the observed color changes.

Arterial ulcers would suggest ischemia, with rest pain, pallor or mottled skin, diminished or absent pulses, and typically less emphasis on allodynia after a fracture. Exertional compartment syndrome causes pain during exercise that worsens with activity and improves with rest, not the persistent burning pain with autonomic color changes seen after injury. Osteomyelitis would present with fever, warmth, swelling, possibly drainage, and systemic inflammatory signs, not the neuropathic-pain–plus–vasomotor-change pattern described.

So the unilateral, severe, burning pain lasting months after fracture with allodynia and color changes most likely represents complex regional pain syndrome.