Regarding the above case, which of the following investigations would be most useful to perform next?

a. Creatinine kinase and myoglobin urinalysis are two useful screens for assessing the likelihood of rhabdomyolysis causing renal failure. Often the diagnosis is self-evident from the history; however, in the elderly and unconscious there may be a lack of clinical information regarding possible muscular damage. A typical scenario for rhabdomyolysis outside of the sphere of trauma is an elderly patient who is brought to hospital with a fall who has been lying on a cold hard floor for hours to days. A renal ultrasound scan is a useful and non-invasive investigation of renal failure without a readily identifiable cause and may show obstruction, renal masses and kidney size that may guide further investigation. Twenty-four-hour urine protein collection is a useful test for quantifying proteinuria, which in the nephrotic syndrome is found in the urine at a rate of 3g/24 hours. Normal range of proteinuria is up to 150mg/24 hours but can be elevated above this level (but below 3g/24 hours) in times of stress, heavy exercise, infection and in children. Bence–Jones protein is an assay for the light-chain breakdown products of monoclonal immunoglobulins that are excreted by the kidney in excess in myeloma. It is commonly encountered in elderly patients who may present with bone pain, signs of hypercalcaemia and even pathological fractures associated with signs of renal failure. Renal artery Dopplers would be a useful investigation of suspected renal artery stenosis (RAS) and allow waveform morphology and flow velocity in the renal and perirenal aorta to be assessed. A reduction in flow below a threshold velocity indicates a significant degree of stenosis. Bilateral RAS is a contraindication to using angiotensin-converting enzyme inhibitors (ACE-I) and renal function must be watched when starting ACE-I in case this condition is present.