Nephrology and Fluid/Electrolyte Disorders
Nephrology questions combine acid-base interpretation, electrolyte correction, and acute versus chronic kidney injury. Candidates must read arterial blood gases systematically: pH, then primary disorder (respiratory vs metabolic), then compensation, then anion gap. Know the causes of acute kidney injury (prerenal, intrinsic, postrenal) and how urine indices and response to fluids separate them. Understand sodium disorders: assess volume status and tonicity before correcting. Master potassium emergencies and the ECG changes of hyperkalemia, plus calcium and magnesium interactions.
Traps: correcting sodium too quickly and causing osmotic demyelination; treating hyperkalemia without checking for hemolysis or pseudohyperkalemia; missing that a "normal" anion gap does not exclude a mixed acid-base disorder; forgetting that diuretics alter urine sodium interpretation.
Worked example: a patient with metabolic acidosis and elevated anion gap. Calculate the gap, then check whether the delta ratio suggests an additional disorder. High anion gap acidosis suggests ketoacidosis, lactic acidosis, renal failure, or toxin. Treatment targets the cause, not just the pH.
Rule of thumb: in any electrolyte disorder, first assess volume status and symptoms, then correct slowly and recheck. Rapid correction is itself a cause of harm.
Key concepts: systematic blood gas reading, acute kidney injury classification, sodium and potassium emergencies, and safe correction rates.