Internal Medicine: Discrimination, Then Treatment
Internal Medicine carries the largest item share and tests probabilistic diagnosis plus guideline-concordant treatment across cardiology, respiratory, gastroenterology, endocrinology, nephrology, haematology and infectious disease. The recurring task is to separate the dangerous cause of a common symptom from the benign one, start the correct first-line therapy, and avoid an intervention that causes harm.
Core rules
- Chest pain. Obtain a 12-lead ECG within 10 minutes. ST elevation triggers immediate reperfusion; a pulse or blood-pressure differential with tearing pain suggests dissection and requires rate control before imaging.
- Dyspnoea. BNP or NT-proBNP plus echocardiography separates cardiac from pulmonary causes; treat the precipitant, not only the congestion.
- Diabetes emergencies. Diabetic ketoacidosis: glucose usually >11 mmol/L, ketonaemia, pH <7.3, bicarbonate <18. Hyperosmolar hyperglycaemic state: glucose >33 mmol/L, osmolality >320 mOsm/kg, minimal ketosis.
- Sodium disorders. Confirm with serum osmolality, assess volume status, and correct chronic hyponatraemia by no more than 8–10 mmol/L in 24 hours.
- Acute kidney injury. Volume status, urinalysis and renal ultrasound come first; fractional excretion of sodium is unreliable during diuretic use.
- Anaemia. Use mean corpuscular volume to branch into microcytic, normocytic and macrocytic pathways.
Worked example
A 24-year-old with type 1 diabetes presents with vomiting, glucose 22 mmol/L, pH 7.18, bicarbonate 9 mmol/L, ketones 3+, and potassium 5.1 mmol/L. This is diabetic ketoacidosis. Start isotonic saline, check potassium, begin an insulin infusion at 0.1 units/kg/h only once potassium is ≥3.3 mmol/L, add dextrose when glucose falls below about 13.9 mmol/L, and replace potassium. The reasoning: total-body potassium is depleted despite a normal or high serum value, and insulin drives potassium into cells, so giving insulin before repletion risks arrhythmia. Find and treat the precipitant, usually infection or insulin omission.
Traps that cost marks
- Giving routine bicarbonate in ketoacidosis; it is reserved for severe acidaemia and worsens hypokalaemia.
- Stopping the insulin infusion when glucose normalises instead of adding dextrose.
- Attributing new dyspnoea in an older patient to age or deconditioning.
- Correcting sodium too quickly and causing osmotic demyelination.