List six causes of bilateral smooth small kidneys.

1. Chronic glomerulonephritis (after acute glomerulonephritis)
2. Generalized arteriosclerosis
3. Atheroembolic renal disease
4. Papillary necrosis
5. Hereditary chronic nephritis (Alport's syndrome)
6. Arterial hypotension (prolonged, dense nephrogram that reverts to normal if hypotension is reversed)