pharmacology quiz 3

A 65-year-old male with chronic atrial fibrillation is on warfarin therapy. He develops tuberculosis and is started on rifampin. After 2 weeks, his INR falls below the therapeutic range despite adherence to warfarin. Which mechanism best explains this drug interaction?

• Displacement of warfarin from plasma proteins

• Increased hepatic metabolism via CYP2C9 induction

• Inhibition of vitamin K epoxide reductase

• Decreased absorption due to intestinal P-glycoprotein blockade

A 32-year-old woman treated with lithium for bipolar disorder presents with nausea, tremors, and confusion after starting hydrochlorothiazide for hypertension. Her lithium levels are markedly elevated. Which mechanism most likely explains this?

• Thiazide diuretics increase lithium tubular reabsorption in the proximal tubule

• Lithium is displaced from albumin binding sites by hydrochlorothiazide

• Hydrochlorothiazide inhibits lithium hepatic metabolism

• Thiazides block lithium's renal secretion through organic anion transporters

A patient on theophylline for COPD experiences severe nausea, vomiting, tachycardia, and seizures after starting erythromycin for pneumonia. Which mechanism underlies this interaction?

• Erythromycin induces CYP1A2, increasing theophylline metabolism

• Erythromycin inhibits CYP3A4, reducing theophylline clearance

• Erythromycin inhibits P-glycoprotein, increasing theophylline absorption

• Erythromycin displaces theophylline from tissue-binding sites

A cancer patient on high-dose methotrexate develops acute kidney injury, leading to dangerously high methotrexate plasma levels. Which therapeutic intervention is most appropriate?

• Administer leucovorin rescue

• Give allopurinol to block xanthine oxidase

• Administer filgrastim to stimulate white cell recovery

• Initiate N-acetylcysteine infusion

In a patient receiving digoxin for heart failure, the addition of amiodarone results in bradycardia, nausea, and visual disturbances. What is the most likely cause?

• Amiodarone displaces digoxin from plasma proteins

• Amiodarone inhibits renal P-glycoprotein-mediated digoxin elimination

• Amiodarone accelerates digoxin metabolism via CYP2D6 induction

• Amiodarone decreases intestinal absorption of digoxin

A renal transplant recipient on cyclosporine develops life-threatening nephrotoxicity after starting ketoconazole for fungal infection. Which mechanism explains this?

• Ketoconazole induces CYP3A4, lowering cyclosporine levels

• Ketoconazole inhibits CYP3A4, increasing cyclosporine levels

• Ketoconazole enhances cyclosporine renal tubular reabsorption

• Ketoconazole increases cyclosporine protein binding in plasma

A patient receiving gentamicin for septicemia develops severe hearing loss and vertigo. Which factor most likely increased his risk of this adverse drug reaction?

• Co-administration with loop diuretics

• Concurrent use of macrolides

• High plasma albumin concentration

• History of asthma

A hypertensive patient on ACE inhibitors develops life-threatening hyperkalemia after being started on spironolactone for heart failure. Which mechanism explains this?

• Both drugs decrease potassium excretion by different pathways

• ACE inhibitors directly stimulate aldosterone secretion

• Spironolactone inhibits the RAAS system more than ACE inhibitors

• Potassium reabsorption is increased in the loop of Henle by both agents

A patient develops severe serotonin syndrome after taking sumatriptan for migraine while on fluoxetine for depression. Which mechanism is responsible?

• CYP3A4 inhibition leading to sumatriptan accumulation

• Additive agonism at serotonin (5-HT1B/1D) receptors

• Inhibition of monoamine oxidase-B by fluoxetine

• Displacement of sumatriptan from plasma proteins by fluoxetine

A patient with epilepsy on phenytoin develops gingival hyperplasia. Which mechanism best explains this adverse drug reaction?

• Increased fibroblast proliferation due to altered collagenase activity

• Direct cytotoxicity to gingival epithelial cells

• Immune-mediated hypersensitivity

• Accumulation of phenytoin metabolites in gingival tissues