PSA Drug Interactions 2

Question 1

A patient stabilized on warfarin begins rifampicin for tuberculosis. What is the most likely effect on the INR?

Accepted Answer

INR decreases due to CYP2C9 induction by rifampicin

Answer

INR increases due to CYP2C9 inhibition by rifampicin

Answer

No change because warfarin is not hepatically metabolized

Answer

INR increases due to rifampicin reducing vitamin K synthesis

Question 2

A patient on simvastatin 40 mg is prescribed clarithromycin for a respiratory infection. What is the primary risk?

Accepted Answer

Increased risk of myopathy or rhabdomyolysis due to elevated simvastatin levels

Answer

Reduced simvastatin efficacy due to enzyme induction

Answer

Decreased clarithromycin absorption due to altered gastric pH

Answer

Cardiac arrhythmia from combined QT prolongation

Question 3

A patient receiving standard-dose codeine reports no pain relief. Which CYP enzyme variant most likely explains this pharmacogenomic interaction?

Accepted Answer

CYP2D6 poor metabolizer preventing conversion of codeine to morphine

Answer

CYP2C9 poor metabolizer reducing codeine clearance

Answer

CYP3A4 ultra-rapid metabolizer accelerating codeine activation

Answer

CYP1A2 poor metabolizer altering codeine glucuronidation

Question 4

Fluoxetine (a potent CYP2D6 inhibitor) is started in a patient already taking tramadol. What is the most important risk?

Accepted Answer

Tramadol accumulation and serotonin syndrome risk due to CYP2D6 inhibition

Answer

Reduced tramadol efficacy from blocked O-desmethylation

Answer

Atorvastatin toxicity via shared CYP2D6 competition

Answer

Warfarin potentiation from CYP2C9 cross-inhibition

Question 5

A patient on combined oral contraceptives starts carbamazepine for epilepsy. What is the pharmacokinetic mechanism of concern?

Accepted Answer

CYP3A4 induction by carbamazepine reduces estrogen and progestogen plasma levels

Answer

CYP3A4 inhibition by carbamazepine increases hormone exposure

Answer

Carbamazepine displaces estrogen from albumin binding sites

Answer

Direct pharmacodynamic antagonism at estrogen receptors

Question 6

Omeprazole co-prescription reduces the antiplatelet efficacy of clopidogrel. What is the mechanism?

Accepted Answer

CYP2C19 inhibition by omeprazole reduces conversion of clopidogrel to its active thiol metabolite

Answer

Pharmacodynamic competition at platelet P2Y12 receptors

Answer

Omeprazole induces P-glycoprotein efflux of clopidogrel from enterocytes

Answer

Alkaline gastric pH from omeprazole reduces clopidogrel dissolution and absorption

Question 7

A patient on long-term methadone maintenance is started on phenytoin for new-onset seizures. What is the anticipated outcome?

Accepted Answer

Opioid withdrawal symptoms from phenytoin-induced increase in methadone clearance

Answer

Phenytoin toxicity from methadone-mediated CYP2C9 inhibition

Answer

Serotonin syndrome from combined opioid and anticonvulsant CNS effects

Answer

Methadone toxicity from decreased hepatic clearance