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 A rise in creatinine of 26 micromol/L in 48 hours, or 1.5 times baseline in seven days, or urine output under 0.5 mL/kg/h for six hours. A postural drop with an empty bladder and a clean dipstick fits.
2. Acute tubular necrosis Follows sustained hypoperfusion or nephrotoxins, with muddy brown casts and a urine sodium above 40 mmol/L. A postural drop with an empty bladder and a clean dipstick makes it worth distinguishing.
3. Pre-renal azotaemia correct Correct. Vomiting, dry membranes, tachycardia and a 25 mmHg postural drop is volume depletion. The empty bladder excludes obstruction and the clean dipstick argues against intrinsic renal disease, which leaves hypoperfusion, the commonest cause of AKI by a wide margin.
4. Glomerulonephritis Red cell casts and proteinuria with hypertension. A postural drop with an empty bladder and a clean dipstick does not show that.
5. Chronic kidney disease A long-standing raised creatinine with small kidneys, anaemia and renal bone disease. A postural drop with an empty bladder and a clean dipstick shows an acute change instead.

The point: Classify by site: pre-renal (hypoperfusion, the commonest), renal (intrinsic damage) or post-renal (obstruction). Every AKI gets a bladder scan and a urine dipstick. Obstruction is the cause you can reverse in minutes. Stop the DAMN drugs: Diuretics, ACE inhibitors and ARBs, Metformin, NSAIDs. Dialyse for refractory hyperkalaemia, acidosis, fluid overload, uraemic complications or poisoning.

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