Basic and Clinical Sciences (Anatomy, Physiology, Pathology, Pharmacology)
What OMRSE tests
Basic and Clinical Sciences integrates anatomy, physiology, pathology, and pharmacology into clinical decisions. Questions often use a drug interaction or mechanism to test whether you understand the underlying principle.
Core rules
- Know the difference between zero-order and first-order elimination; phenytoin, ethanol, and high-dose aspirin show saturation.
- Cytochrome P450 induction reduces drug levels; inhibition raises them. Rifampicin is a strong inducer; azole antifungals and macrolides are inhibitors.
- Warfarin acts on vitamin K-dependent clotting factors and is monitored by INR; many interactions are pharmacokinetic or pharmacodynamic.
- Volume of distribution determines loading dose; clearance determines maintenance dose.
Worked example
A 68-year-old with atrial fibrillation is stable on warfarin with INR 2.5. He starts rifampicin for tuberculosis. After 10 days, INR falls to 1.2 and he develops a deep vein thrombosis. Rifampicin induces CYP2C9 and increases warfarin metabolism, lowering INR and anticoagulant effect. The correct response is to anticipate the interaction, monitor INR more frequently, and use an alternative anticoagulant or adjust warfarin with specialist input. Do not simply stop warfarin without a plan.
Traps that cost marks
- Confusing enzyme induction with inhibition.
- Assuming an interaction is immediate rather than over days.
- Monitoring INR weekly when starting a strong inducer.
- Attributing a low INR to diet alone.