Internal Medicine: Reasoning from Mechanism to Management
Internal Medicine questions reward a disciplined sequence: identify the syndrome, localize the process, then choose the single best next step. Master the high-yield clusters - acid-base disorders, electrolyte abnormalities, ischemic heart disease, heart failure, arrhythmias, pneumonia, thromboembolism, diabetes, thyroid disease, chronic kidney disease, and cirrhosis. For each, know the defining pathophysiology, because SMLE items often test the mechanism rather than the label.
Rules of thumb that pay off
- Acid-base: calculate the anion gap and the expected compensation before naming a mixed disorder. A delta ratio (delta gap / delta bicarbonate) distinguishes pure high-anion-gap acidosis from a superimposed metabolic alkalosis or normal-gap acidosis.
- Chest pain: the ECG and troponin define the pathway; ST elevation triggers reperfusion, not observation.
- Dyspnea with hypoxia: risk-stratify for pulmonary embolism using pretest probability, not D-dimer alone.
- Hyponatremia: assess volume status and urine osmolality; correct slowly to avoid osmotic demyelination.
Traps
Questions often offer a correct-but-premature investigation when the stem demands immediate stabilization. Watch for distractors that treat a number rather than a patient (e.g., aggressively lowering sodium or glucose). Beware "classic" presentations that omit the key negative - a normal chest radiograph does not exclude early pneumonia or PE. Also distinguish disease-modifying therapy from symptomatic therapy.
Worked example
A patient with vomiting has pH 7.50, PaCO2 46 mmHg, HCO3 34 mEq/L. Metabolic alkalosis is present; the expected PaCO2 rise (~0.7 per 1 mEq/L HCO3 rise) is roughly 7 mmHg above 40, i.e., ~47 - so compensation is appropriate and no second disorder is present. The next step is volume and chloride repletion, not acetazolamide.