Nephrology and Fluid-Electrolyte Disorders
Core concepts
Understand the AKI workup: prerenal vs intrinsic vs postrenal, the fractional excretion of sodium (low in prerenal, high in ATN), urinalysis with muddy-brown casts in ATN, and the indications for urgent dialysis (AEIOU: acidosis, electrolytes, intoxication, overload, uremia). Know CKD staging by eGFR and albuminuria, the role of ACE inhibitors/SGLT2 inhibitors in slowing progression, and the management of hyperkalemia, hyponatremia (assess volume status and urine osmolality), and acid-base disorders using the anion gap and Winter's formula.
Traps
Examiners test the difference between SIADH (euvolemic, low serum osmolality, high urine osmolality, urine sodium >40) and hypovolemic hyponatremia (correct with saline, not fluid restriction). Beware of overcorrecting sodium (osmotic demyelination) and of the "normal anion gap" acidosis (diarrhea, RTA). Remember that a normal creatinine does not exclude early AKI in a patient with low muscle mass, and that nephrotoxins (NSAIDs, contrast, aminoglycosides) are common culprits.
Worked rule of thumb
Hyponatremia: check serum osmolality first. If low, check volume status. Hypovolemic → isotonic saline. Euvolemic with concentrated urine → SIADH → fluid restriction. Correct sodium slowly (≤8–10 mmol/L per 24 h) to avoid osmotic demyelination.
Key points
- FENa <1% favors prerenal; >2% favors ATN.
- Urgent dialysis indications: AEIOU.
- SIADH is euvolemic hyponatremia with concentrated urine.
- Correct chronic hyponatremia slowly.
- SGLT2 inhibitors and ACE inhibitors slow CKD progression.