List three causes of a unilateral small scarred kidney.

1. Reflux nephropathy (cortical destruction over retracted papilla with dilated calix; polar initially)
2. Lobar infarction (broad based, over normal papilla; embolic, usually cardiac [e.g., atrial fibrillation, subacute bacterial endocarditis])
3. Tuberculosis (scarring with retraction of underlying papilla)