<prompt> Radiologically, the earliest sign of osteomyelitis is -Loss of muscle and fat planesPresence of sequestrumCallus formationPeriosteal reaction  <response>: Osteomyelitis is the infection of bone by microorganisms. It can be acute or chronic. Acute osteomyelitis: This can be primary haematogenousor secondaryfollowing open fracture or bone operation. Staphylococcus aureus is the commonest causative organism. Cf: 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. 