Pharmacology principles & monitoring flashcards
17 high-yield MRCP Part 1 cards from the Intellect Medicos Basic sciences & statistics course. Try to answer each question before you open it — then study them properly with spaced repetition, which brings each card back just before you would forget it.
Study these 17 cards Save progress — free account
-
Name three drugs that follow zero-order (saturable) elimination kinetics, at least at higher doses.
Show answer
Phenytoin, ethanol (alcohol) and salicylates (aspirin) at high dose.
Why? In zero-order kinetics the elimination enzymes are saturated, so a constant AMOUNT (not proportion) is removed per unit time and the half-life is not constant.
-
A man on phenytoin 300 mg daily has a subtherapeutic level, so the dose is increased to 350 mg. Two weeks later he is ataxic with nystagmus. Which pharmacokinetic property explains the disproportionate rise in his level?
Show answer
Saturable (zero-order) metabolism: near saturation, a small dose increase causes a large rise in plasma concentration.
Why? Phenytoin moves from first-order to zero-order kinetics within the therapeutic range, so levels climb steeply once the enzymes saturate.
-
In first-order kinetics a constant ______ of drug is eliminated per unit time, and steady state is reached after about 4–5 half-lives.
Show answer
proportion
Why? Most drugs follow first-order kinetics, so the half-life stays constant whatever the dose; the 4–5 half-life rule also tells you how long a drug takes to wash out.
-
In first-order kinetics a constant proportion of drug is eliminated per unit time, and steady state is reached after about ______.
Show answer
4–5 half-lives
Why? Most drugs follow first-order kinetics, so the half-life stays constant whatever the dose; the 4–5 half-life rule also tells you how long a drug takes to wash out.
-
Which pharmacokinetic parameter determines the loading dose, and which determines the maintenance dose?
Show answer
Loading dose: volume of distribution. Maintenance dose: clearance.
Why? A loading dose fills the volume the drug distributes into; the maintenance dose replaces what is cleared each dosing interval.
-
Name three drugs or substances that induce cytochrome P450 enzymes.
Show answer
Any three: rifampicin, carbamazepine, phenytoin, phenobarbital, St John's wort, chronic alcohol excess, griseofulvin; smoking induces CYP1A2.
Why? Inducers speed up metabolism of other drugs (e.g. warfarin, the pill, ciclosporin), lowering their levels and effect.
-
Name three drugs or substances that inhibit cytochrome P450 enzymes.
Show answer
Any three: clarithromycin/erythromycin, ciprofloxacin, fluconazole/ketoconazole, sodium valproate, isoniazid, amiodarone, omeprazole, cimetidine, grapefruit juice, acute alcohol intake.
Why? Inhibitors slow metabolism of other drugs, raising their levels and toxicity (e.g. warfarin bleeding, statin myopathy).
-
A 26-year-old taking the combined oral contraceptive pill is starting rifampicin. What contraceptive advice should she be given?
Show answer
Rifampicin (a potent enzyme inducer) makes the pill unreliable: switch to a method it does not affect (copper IUD, LNG-IUS or DMPA injection). The pill should not be relied on during treatment or for 28 days after stopping rifampicin.
Why? Enzyme induction speeds oestrogen and progestogen metabolism, so contraceptive levels fall; the effect lasts about 4 weeks after the inducer stops.
-
A man on simvastatin 40 mg is prescribed clarithromycin for a chest infection. What is the risk, and what should be done?
Show answer
Myopathy or rhabdomyolysis (clarithromycin inhibits CYP3A4 and raises simvastatin levels): withhold simvastatin for the course.
Why? Simvastatin and atorvastatin are CYP3A4 substrates. The BNF rates clarithromycin with simvastatin as a severe interaction and advises avoiding the combination.
-
A man with COPD on oral theophylline stops smoking. Two weeks later he has nausea, palpitations and a tachycardia. What is the most likely explanation?
Show answer
Theophylline toxicity: smoking induces CYP1A2, so stopping smoking reduces theophylline clearance and its level rises.
Why? Loss of the enzyme-inducing effect of smoke raises levels of CYP1A2 substrates within days to weeks.
-
Name four drugs with a narrow therapeutic index that are monitored by plasma levels.
Show answer
Any four: lithium, digoxin, phenytoin, theophylline, aminoglycosides (gentamicin), vancomycin, ciclosporin, tacrolimus.
Why? A narrow therapeutic index means the toxic dose is close to the effective dose, so small changes in level matter.
-
How long after the dose should a lithium level be taken?
Show answer
12 hours post-dose.
Why? Lithium has a narrow therapeutic index; a standardised 12-hour level allows comparison with the target range.
-
How long after the dose should a digoxin level be taken?
Show answer
At least 6 hours post-dose.
Why? Digoxin needs about 6 hours to distribute into tissues; earlier samples overestimate the effective level.
-
Slow acetylator status increases the risk of drug-induced lupus with which three drugs?
Show answer
Hydralazine, procainamide and isoniazid.
Why? These drugs are cleared by N-acetyltransferase; slow acetylators accumulate them. Drug-induced lupus typically has antihistone antibodies.
-
Reactivation of which latent infection is the classic serious risk of anti-TNF drugs such as infliximab or adalimumab, so it must be screened for before starting?
Show answer
Tuberculosis (latent or active).
Why? TNF is essential for granuloma maintenance, so blocking it can reactivate latent TB.
-
Which vitamin is co-prescribed with isoniazid, and which adverse effect does it prevent?
Show answer
Pyridoxine (vitamin B6), to prevent peripheral neuropathy.
Why? Isoniazid causes functional B6 deficiency. B6 is also needed to make niacin from tryptophan, so isoniazid can rarely cause pellagra.
-
Which enzyme does warfarin inhibit, and which clotting factors are affected?
Show answer
Vitamin K epoxide reductase; gamma-carboxylation of factors II, VII, IX and X (and proteins C and S) is reduced.
Why? Vitamin K is the cofactor for gamma-carboxylation of these factors. Warfarin stops vitamin K being recycled, so the factors cannot bind calcium and work.