Internal Medicine: Core Reasoning and Common Traps
What the candidate must understand
Internal medicine questions test pattern recognition plus pathophysiology. Anchor your revision on the major syndromic clusters: ischemic and structural heart disease, heart failure with reduced versus preserved ejection fraction, arrhythmias, chronic obstructive and restrictive lung disease, acute kidney injury versus chronic kidney disease, electrolyte and acid–base disorders, diabetes and its microvascular complications, thyroid disease, anemia classified by mean corpuscular volume, and thromboembolic disease. Know the diagnostic criteria that trigger a management change: for example, the combination of symptoms, signs, and natriuretic peptide levels in heart failure, or the anion gap and delta gap in metabolic acidosis.
Traps that appear in questions
- Confusing the cause of a metabolic acidosis with the compensation; always check whether respiratory compensation is appropriate using Winter's formula.
- Attributing anemia to iron deficiency without confirming low ferritin; ferritin rises as an acute-phase reactant.
- Choosing a treatment before excluding a contraindication (for example, beta-blockade in decompensated heart failure or in high-grade atrioventricular block).
- Missing secondary causes of hypertension or of osteoporosis when red flags are given.
- Ignoring drug-induced disease: cough from ACE inhibitors, hyperkalemia from renin–angiotensin blockade, hypoglycemia from sulfonylureas.
Worked example: the anion gap
A patient has pH 7.20, bicarbonate 10 mmol/L, sodium 140, chloride 100. Anion gap = 140 − (100 + 10) = 30, clearly elevated. The delta ratio = (30 − 12)/(24 − 10) ≈ 1.3, which points to a pure high-anion-gap acidosis rather than a mixed disorder. Use this arithmetic every time rather than guessing from the clinical picture.
Rule of thumb
When a question asks for the next best step, prefer the test or action that changes immediate management and is least invasive; when it asks for the most likely diagnosis, weigh prevalence and the discriminating feature named in the stem.