From first question to exam-ready.

A 76 year old man has been vomiting for three days with poor oral intake. His creatinine is 240 micromol/L against a baseline of 88. He is clinically dehydrated with dry mucous membranes, a pulse of 104 beats/minute and a postural drop of 25 mmHg. His bladder scan shows 30 mL. Urine dipstick is negative for blood and protein. What is the SINGLE most likely diagnosis?

1. Acute kidney injury The blood test for kidney function (creatinine) rises by 26 micromol/L in 48 hours, or 1.5 times the normal level in seven days, or the amount of urine made falls below 0.5 mL for each kg of body weight per hour for six hours. That does not match a drop in blood pressure on standing, with an empty bladder and a clean urine test.
2. Acute tubular necrosis Happens after a long spell of low blood flow to the kidney or after drugs that harm the kidney (nephrotoxins), with muddy brown casts (bits of dead cells) in the urine and a urine sodium above 40 mmol/L. It does not fit a drop in blood pressure on standing, with an empty bladder and a clean urine test.
3. Pre-renal azotaemia correct Correct. Vomiting, a dry mouth, a fast heart rate and a 25 mmHg drop in blood pressure on standing all point to the body running low on fluid. The empty bladder rules out a blockage, and the clean urine test argues against a kidney problem inside the kidney itself. That leaves low blood flow to the kidney, by far the most common cause of acute kidney injury.
4. Glomerulonephritis Red blood cells clumped together (red cell casts) and protein in the urine (proteinuria), with high blood pressure. Here, a drop in blood pressure on standing, with an empty bladder and a clean urine test points elsewhere.
5. Chronic kidney disease A kidney problem that has been there a long time, with a raised creatinine, small kidneys, a low blood count (anaemia) and bone disease caused by the kidneys. But a drop in blood pressure on standing, with an empty bladder and a clean urine test does not point to it.

The point: Sort the cause by where it sits. Pre-renal means the blood flow reaching the kidney is too low (hypoperfusion), and this is the most common type. Renal means the kidney itself is damaged (intrinsic damage). Post-renal means something is blocking the flow of urine (obstruction). Give every patient with acute kidney injury (AKI) a bladder scan and a urine dipstick test. A blockage is the one cause you can fix in minutes. Stop the DAMN drugs: Diuretics (water tablets), ACE inhibitors and ARBs (blood pressure drugs), Metformin (a diabetes drug) and NSAIDs (anti-inflammatory painkillers). Give dialysis (a machine that cleans the blood) for a high potassium level that will not come down, blood that is too acidic, too much fluid in the body, problems from too much waste in the blood, or poisoning.

Source: NICE NG148 — Acute kidney injury: prevention, detection and management NICE · tier 1, national regulator or guidance