Nephrology and Fluid-Electrolyte Disorders
This domain rewards a systematic approach: classify the acute kidney injury, look at the urine, count the anion gap, and calculate the osmolality before you name a syndrome. Both the FRCP-style and Egyptian board formats favour short calculations followed by management choices.
Acute kidney injury
- KDIGO staging uses creatinine rise (0.3 mg/dL in 48 hours, or 1.5 times baseline in 7 days) and urine output below 0.5 mL/kg/h for six hours.
- Prerenal states show fractional excretion of sodium below 1 percent, urine sodium below 20, urine osmolality above 500, and bland sediment; acute tubular necrosis shows fractional excretion above 2 percent, muddy brown casts, and urine osmolality near 300.
- Remove nephrotoxins, restore perfusion, and treat the cause. Dialyse for AEIOU: acidosis, electrolyte disturbance (refractory hyperkalaemia), intoxication, overload, uraemia (encephalopathy, pericarditis).
Sodium and potassium
- Assess tonicity first in hyponatraemia, then urine osmolality, then urine sodium. Hypotonic, euvolemic, urine osmolality above 100 and urine sodium above 30 suggests SIADH: treat with fluid restriction, treat the cause, and use hypertonic saline only for seizures.
- Correct chronic hyponatraemia by no more than 8-10 mmol/L in 24 hours to avoid osmotic demyelination.
- Hyperkalaemia with ECG changes: intravenous calcium first, then insulin with glucose, salbutamol, and definitive removal with a binder or dialysis.
Acid-base and glomerular disease
- High anion gap acidosis: methanol, uraemia, diabetic ketoacidosis, paraldehyde, isoniazid, lactic acidosis, ethylene glycol, salicylates.
- Nephrotic syndrome: proteinuria above 3.5 g/day, albumin below 30 g/L, oedema, hyperlipidaemia. Nephritic: haematuria with red cell casts, hypertension, oliguria.
- CKD is staged on eGFR with G1-G5 and albuminuria A1-A3; use the CKD-EPI equation and reassess after three months.
Worked example
A 70-year-old on an ACE inhibitor and indomethacin for osteoarthritis presents with creatinine 2.6 mg/dL (baseline 1.0), urine sodium 12 mmol/L, fractional excretion of sodium 0.4 percent, and no casts. This is prerenal acute kidney injury from combined haemodynamic insult; stop both drugs, give careful isotonic fluid if not overloaded, and recheck creatinine and potassium in 24-48 hours rather than dialysing.
Traps
Giving potassium in any form during hyperkalaemia treatment with insulin; assuming a normal anion gap excludes acidosis; correcting sodium too fast; and calling every rising creatinine 'acute tubular necrosis' without checking the urine indices.