What is focal region pyelonephritis?

A localised area. A localised area of inflammation of the kidney could be described as focal region pyelonephritis. Upper and lower poles of kidney have greater propensity for intrarenal reflux (to allow bacteria ti attack kidney tuissue) and be focal regions for pyelonephritis. An infected kidney stone could be the center of a focal region of pyelonephritis.
Localized infection. It is an infection of a part of the kidney, not involving the entire kidney.