**Core Concept**
Periosteal reaction is a radiological sign of bone inflammation, typically seen in acute osteomyelitis. It reflects the body’s response to infection, where the periosteum becomes activated and forms new bone due to inflammatory stimuli.

**Why the Correct Answer is Right**
In acute osteomyelitis, periosteal reaction appears earliest around **10 days** post-onset. This timing coincides with the peak of inflammatory activity, where bacterial invasion triggers osteoblastic proliferation at the bone surface. The periosteum responds by forming new bone, visible on X-rays as a "cloud-like" or "codfish" appearance. This phase follows the initial septic infiltration and occurs before significant bone destruction or sclerosis.

**Why Each Wrong Option is Incorrect**
Option A: 5 days – Early bone changes like soft tissue swelling or bone marrow infiltration occur at this stage, but periosteal reaction is not yet visible.
Option C: 15 days – By this time, the reaction may be more pronounced, but it is not the earliest appearance.
Option D: 20 days – This is too late; by now, the infection may have progressed to chronic phase or bone destruction, and periosteal reaction may be obscured.

**Clinical Pearl / High-Yield Fact**
Periosteal reaction appears earliest at **10 days** in acute osteomyelitis and is a key radiological sign distinguishing it from other bone pathologies like tumors or avascular necrosis. Always correlate imaging with clinical signs like fever, pain, and localized tenderness.

✓ Correct Answer: B. 10 days