Renal and Urinary System
Physiology and acute kidney injury
Glomerular filtration rate is best estimated by clearance; inulin is the gold standard and creatinine is the practical marker. Fractional excretion of sodium below 1% with oliguria suggests prerenal physiology; above 2% suggests acute tubular necrosis with muddy brown casts. Use fractional excretion of urea when the patient is on diuretics. Postrenal causes are identified by bladder scan and renal ultrasound. Prerenal injury responds to volume; acute tubular necrosis requires supportive care.
Glomerular disease
Nephritic: hematuria with red cell casts, hypertension, edema, and oliguria - post-streptococcal glomerulonephritis, IgA nephropathy, lupus nephritis, and rapidly progressive glomerulonephritis (anti-GBM, ANCA). Nephrotic: proteinuria above 3.5 g/day with hypoalbuminemia, edema, and hyperlipidemia - minimal change disease (children, foot process effacement), focal segmental glomerulosclerosis (HIV, obesity), membranous nephropathy (anti-PLA2R), diabetic nephropathy, and amyloidosis.
Fluids, electrolytes, acid-base
Hyponatremia: assess volume status and urine osmolality; correct by less than 8 mEq/L per day to avoid osmotic demyelination. Hyperkalemia: obtain an ECG first (peaked T waves, widening QRS), then give calcium gluconate, insulin with glucose, a beta-agonist, bicarbonate, and dialysis if refractory. High anion gap acidosis has a MUDPILES differential; normal gap acidosis suggests diarrhea or renal tubular acidosis (type 1 and type 2 with hypokalemia, type 4 with hyperkalemia). Loop diuretics block NKCC2 in the thick ascending limb, thiazides block NCC in the distal convoluted tubule, spironolactone blocks the aldosterone receptor, acetazolamide blocks carbonic anhydrase.
Stones and infection
Calcium oxalate stones are most common; struvite stones form staghorns with urease-positive bacteria; uric acid stones are radiolucent and respond to urine alkalinization. Urinalysis shows nitrite and leukocyte esterase in urinary tract infection; pyelonephritis adds fever and flank pain.
Rule of thumb
When creatinine rises, check the urine sediment alongside the fractional excretion of sodium - casts often separate prerenal from intrinsic disease faster than the numbers do.