Unilateral renal agenesis and the congenital solitary functioning kidney: developmental, genetic and clinical perspectives

based on > 9000 (postnatal) autopsies. However, in clinical practice the clinician is generally faced with making a radiological diagnosis of ‘URA’ in a child or adult, or perhaps in a fetus in which just one kidney has been visualized on ultrasonography (US) during mid-gestation. Using US, URA can be ‘missed’ in the fetus or neonate because the adrenal, which then occupies the renal bed, can be mistaken for a kidney [6]. Using US, Roodhooft et al. [7] reported that URA occurred in ≈ 1 : 500 of the general population, which is more frequent than the autopsy value cited above [5]. There are caveats about using an ‘empty renal bed’ assessed by US to diagnose URA. Renal ectopia (e.g. pelvic and cross-fused) kidneys should be considered and sought by US [8] and, if necessary, by functional scanning with IVU and/or isotope renography (e.g. with 99m Tc-DMSA, concentrated in functioning tubules). Furthermore, all these radiological techniques might fail to detect a very small ( < 2 cm across) contralateral kidney affected by renal ‘aplasia’ (a tiny, dysplastic organ, containing undifferentiated and metaplastic tissues) or atrophy secondary to renal artery stenosis or neonatal renal venous thrombosis [9–11]. Such ‘remnants’, confirmed at laparotomy, have rarely been implicated in causing hypertension [10,12]; MRI might prove a sensitive technique with which to find such rudiments [10].