Prerenal acute kidney injury (AKI) has a history or signs of circulatory collapse. If AKI is due to renal causes, there is salt and water retention with blood, protein and casts in the urine, and symptoms specific to an accompanying disease. An ultrasound is the most important investigation, in order to identify obstruction of the renal tract that may require intervention, small kidneys of chronic kidney disease, or large echobright kidneys with loss of corticomedullary differentiation, typical of an acute process. Percutaneous renal biopsy is indicated if the diagnosis is unclear, in order to exclude a crescentic nephritis that would require treatment with immunosuppression. There is an overlap with thrombotic thrombocytopenic purpura, which is also characterised by thrombocytopenia, microangiopathic haemolytic anaemia and abnormalities in renal function. Renal dysplasia is very commonly associated with abnormalities of the lower urinary tract including obstructive lesions. Renal dysplasia is suggested on ultrasound scanning by the visualisation of irregular-shaped organs with loss of corticomedullary differentiation.