Pharmacology
Pharmacokinetics and drug action
Pharmacology questions often start with pharmacokinetics: absorption, bioavailability, volume of distribution, clearance, half-life, and metabolism. First-order elimination means a constant fraction is eliminated per unit time; zero-order means a constant amount is eliminated, so small dose increases can cause toxicity. Pharmacodynamics includes receptor types, agonists, antagonists, partial agonists, and dose-response curves. Autonomic pharmacology is high yield: know muscarinic and nicotinic effects, alpha and beta receptors, and the drugs that act at each. Antimicrobials require mechanism and resistance patterns.
Traps and worked rules
A common trap is confusing half-life with duration of effect. Another is forgetting that lipid-soluble weak bases can be trapped in acidic compartments, altering distribution. Cytochrome P450 inhibitors and inducers are frequently tested; inhibitors generally raise drug levels, inducers lower them. For loading dose, use loading dose = (Vd × target concentration) / bioavailability. For maintenance dosing, clearance and bioavailability determine the rate needed.
Worked example
If a drug has a large volume of distribution, a large loading dose may be needed to reach therapeutic concentration even though the maintenance dose depends mainly on clearance. Do not assume a large Vd means rapid elimination. The rule of thumb: loading dose reflects Vd; maintenance dose reflects clearance.