Radiologically, the earliest sign of osteomyelitis is -: (A) Loss of muscle and fat planes, (B) Presence of sequestrum, (C) Callus formation, (D) Periosteal reaction
Answer is D. Osteomyelitis is the infection of bone by microorganisms. It can be acute or chronic. 
Acute osteomyelitis: This can be primary (haematogenous)or secondary(following open fracture or bone operation). Staphylococcus aureus is the commonest causative organism. C/f: Child presents with an acute onset of pain and swelling at end of a bone, associated with systemic features of infection like a fever. 
Investigations: Blood- polymorphonuclear leucocytosis and an elevated ESR. X-rays - The earliest sign to appear in X-ray is a periosteal new bone deposition at metaphysics. This is called periosteal reaction. It takes about 7-10 days to appear. Bone scan- bone scan using Tc 99 may show increased uptake by bone in the metaphysis.