Practice questions › Davidson's Principles of Medicine › Davidson's Medicine - Part 1: Fundamentals of Medicine
02 Clinical therapeutics and good prescribing MCQs
Davidson's Medicine - Part 1: Fundamentals of Medicine · Davidson's Principles of Medicine · 13 practice questions
- Drug interactions
- ADR classification
- Metabolism
- Timing of drug therapy
- Prescribing in special circumstances
- Volume of distribution
- Hypersensitivity
- Therapeutic index
1. A man on warfarin is given clarithromycin and his INR rises; another on ciclosporin starts St John's wort and rejects a graft; a third on lithium starts a thiazide and develops tremor. Which mechanisms fit in order?
- ADisplacement from binding, enzyme induction, enhanced excretion
- BReduced metabolism, enzyme induction, reduced renal elimination
- CEnzyme induction, reduced metabolism, reduced absorption
- DReduced absorption, reduced renal elimination, enzyme induction
Show answer
B. Reduced metabolism, enzyme induction, reduced renal elimination Macrolides inhibit CYP metabolism, St John's wort induces it, diuretics reduce lithium clearance.
2. A drug has an ED50 of 0.1 mg for its beneficial effect and 100 mg for its most important adverse effect. A second drug has ED50 values of 10 mg and 40 mg. Which statement compares their prescribing margins correctly?
- AThe second has a higher index because its doses are larger
- BThe indices cannot be compared without the volume of distribution
- CBoth have identical indices because the drugs are similar
- DThe first has a therapeutic index of 1000 and the second 4, so the first gives far more room in choosing a dose
Show answer
D. The first has a therapeutic index of 1000 and the second 4, so the first gives far more room in choosing a dose Therapeutic index is the ratio of ED50 for the adverse effect to that for the beneficial effect (1000 versus 4).
3. A patient with a long-half-life drug needs therapeutic levels within hours rather than after five half-lives. A colleague suggests the same daily dose but split into smaller more frequent doses. Which is the better strategy and why?
- AA modified-release formulation, because it accelerates absorption
- BSplitting the maintenance dose, because it shortens time to plateau
- CA loading dose, because it reaches a concentration near the plateau without waiting for steady state
- DDoubling the interval, because it lowers peaks
Show answer
C. A loading dose, because it reaches a concentration near the plateau without waiting for steady state A loading dose is much larger than the maintenance dose and equals the amount needed in the body at steady state.
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