Basic Medical Sciences: Mechanisms That Change Management
Basic Medical Sciences on an integrated licensing paper is tested through clinical vignettes rather than isolated recall: you must explain a finding with physiology or pathology, predict a drug effect with pharmacokinetics, or interpret a laboratory pattern. The exam favours mechanisms that alter management, so attach a clinical consequence to every fact you learn.
Core rules
- Homeostasis and compensation. Read pH before PaCO2 before bicarbonate. Compensation never fully corrects the primary disorder and never overshoots it.
- Structure–function localisation. A lesion produces a deficit predictable from anatomy: a named cranial nerve palsy, a sensory level, a vascular territory.
- Inflammation and repair. Acute inflammation is neutrophil-predominant and vascular; chronic inflammation is lymphocyte and macrophage-predominant with fibrosis and granulomas.
- Kinetics. For first-order drugs, half-life = 0.693 × Vd / clearance; steady state is reached in 4–5 half-lives; loading dose = Vd × target concentration / bioavailability.
- Acid–base arithmetic. Calculate the anion gap (Na − [Cl + HCO3]); when it is raised, calculate the delta ratio to detect a mixed disorder.
Worked example
A 70-kg man receives 500 mg of drug X by intravenous bolus. Vd is 0.5 L/kg and clearance is 3.5 L/h. Vd = 35 L, so peak concentration = 500/35 ≈ 14.3 mg/L. Half-life = 0.693 × 35/3.5 ≈ 6.9 h, so steady state on repeated dosing is reached at about 5 half-lives ≈ 35 h. To hold the same average concentration by infusion, rate = clearance × concentration = 3.5 × 14.3 ≈ 50 mg/h. The reasoning to carry into any kinetics item: Vd sets the loading dose, clearance sets the maintenance dose, and renal or hepatic impairment changes clearance first.
Traps that cost marks
- Swapping the roles of Vd and clearance, which inverts loading and maintenance answers.
- Assuming saturable (zero-order) kinetics for a first-order drug; phenytoin and ethanol are the standard exceptions.
- Accepting a "normal" PaCO2 in a tachypnoeic acidotic patient as reassuring - it suggests exhaustion or a second disorder.
- Forgetting unit conversions between mg/L and µg/mL, or between mmol/L and mEq/L.