**3.2.1 Posterior urethral valves**

PUV, also known as *congenital obstructing posterior urethral membrane*, is found in 1 out of 5000-8000 live births, and occurs only in males (King, 1985). Oligohydramnios is a common consequence, and renal dysplasia may also be present. Using postnatal fluoroscopic VCUG, the gold standard for diagnosis of PUV and other lower urinary tract obstructions (Riedmiller et al., 2001), the classic finding for PUV is a linear filling defect in the column of radiocontrast filling a markedly dilated posterior urethra. However, this distinct linear radiolucent band corresponding to the "valve" is not always seen, because the obstructing membrane can become distended and take on a more sail-like or windsock appearance, as shown in Figure 4. Secondary VUR is found in 25-50% of PUV cases (Agarwal, 1999). In a subset of patients, unilateral VUR may provide a "pop-off" effect, whereby renal tissue and function on the non-refluxing side is preserved at the expense of severe dysplasia and dysfunction in the refluxing kidney (Greenfield et al., 1983).
