Internal Medicine: Reasoning Across Systems
Priority concepts
High-yield clusters on this exam are cardiopulmonary diagnosis, acid–base and electrolyte interpretation, glycaemic emergencies, acute kidney injury, anaemia classification, and the four-pillar heart-failure regimen (ARNI/ACE inhibitor, beta-blocker, mineralocorticoid antagonist, SGLT2 inhibitor). Know the diagnostic criteria you are being tested on rather than isolated facts: for example, sepsis is infection plus organ dysfunction, and heart failure is symptoms plus objective evidence, not a single natriuretic peptide value.
Acid–base discipline
Always calculate three things: the anion gap (Na − Cl − HCO3), the albumin-corrected gap (add 2.5 × [4.0 − albumin] when albumin is low), and expected compensation (Winters: PaCO2 = 1.5 × HCO3 + 8 ± 2 for metabolic acidosis). The delta ratio (AG − 12)/(24 − HCO3) reveals a second metabolic process when it departs from roughly 1.
Traps
Questions reward the next best step, not the definitive diagnosis. A normal anion gap does not exclude a gap acidosis when albumin is low. A reassuring respiratory rate in an obviously tachypnoeic patient should prompt a blood gas, not reassurance. In diabetic ketoacidosis, serum potassium is normal or high despite total-body depletion - check K⁺ before starting insulin, and never give bicarbonate routinely.
Worked example
pH 7.30, PaCO2 30 mmHg, HCO3 14 mmol/L, Na 140, Cl 105, normal albumin. Gap = 21 (raised). Expected PaCO2 = 1.5 × 14 + 8 = 29 ± 2; measured 30, so respiratory compensation is appropriate. Delta ratio = 9/10 ≈ 0.9, consistent with a single high-gap acidosis. Treat the cause and support the patient.
Rule of thumb
High-gap acidosis suggests organic acids or toxins; normal-gap acidosis suggests gastrointestinal bicarbonate loss or renal tubular acidosis; if compensation does not fit the formula, look for a second disorder.