Medicine and Allied Disciplines: pattern recognition and the numbers behind it
Internal medicine items test whether a presentation can be converted into a syndrome, then into a diagnosis, then into a first action. The exam favours common conditions with crisp criteria, plus acid-base and ECG interpretation, which are scored by rule rather than by memory.
Core rules
- Diabetes: fasting plasma glucose 7.0 mmol/L or more, 2-hour value in an oral glucose tolerance test 11.1 mmol/L or more, or HbA1c 6.5% or more; symptoms plus a random glucose of 11.1 mmol/L or more also diagnose.
- Nephrotic syndrome: proteinuria above 3.5 g/day, hypoalbuminaemia, oedema, hyperlipidaemia. Nephritic syndrome: haematuria with red cell casts, hypertension, oliguria.
- Acid-base: check pH first, then whether carbon dioxide and bicarbonate move in the direction that compensates, then calculate the anion gap = sodium - (chloride + bicarbonate), normal 8-12 mmol/L.
- Heart failure with reduced ejection fraction is defined by ejection fraction below 40%; the pillar treatments are renin-angiotensin blockade, beta-blockade, mineralocorticoid antagonism and SGLT2 inhibition.
Worked example
Arterial blood gas: pH 7.28, PaCO2 30 mmHg, bicarbonate 14 mmol/L, sodium 140, chloride 100. The pH is acidotic, so the primary process is acidosis. Bicarbonate is low, confirming a metabolic acidosis; the PaCO2 of 30 is a compensatory respiratory alkalosis. Anion gap = 140 - (100 + 14) = 26, so this is a high-anion-gap acidosis, generating a differential of methanol, uraemia, diabetic ketoacidosis, propylene glycol, isoniazid, lactate, ethylene glycol and salicylates. The next step is to identify the cause, not to give bicarbonate reflexively.
Traps that cost marks
- Calling a normal PaCO2 adequate compensation when it should have fallen further.
- Confusing the anion gap with the delta ratio; the delta ratio distinguishes mixed disorders.
- Diagnosing nephrotic syndrome from dipstick protein alone without quantification.
- Missing that a normal anion gap does not exclude a serious metabolic acidosis.