The reasoning
Routes: haematogenous (children: metaphysis of long bones, slow sinusoidal flow; adults: spine), contiguous (diabetic foot, pressure sores), direct inoculation (open fracture, surgery). Organisms: *S. aureus (commonest overall); Salmonella in sickle cell disease (S. aureus still commonest); Pseudomonas* (puncture through shoes); TB (spine).