CAOP Drug Interactions and Contraindications 2

Question 1

A patient stabilised on lithium for bipolar disorder is prescribed naproxen for arthritis. What is the most likely outcome?

Accepted Answer

Lithium levels increase due to reduced renal prostaglandin-mediated excretion

Answer

Lithium levels decrease due to increased renal clearance

Answer

Naproxen levels increase due to lithium-mediated protein binding displacement

Answer

No clinically relevant interaction is expected

Question 2

Beta-blockers are contraindicated in patients with which of the following respiratory conditions?

Accepted Answer

Reactive airway disease / bronchial asthma

Answer

Mild seasonal allergic rhinitis

Answer

Obstructive sleep apnoea

Answer

Pulmonary hypertension

Question 3

Concomitant use of clopidogrel and omeprazole is clinically significant because:

Accepted Answer

Omeprazole inhibits CYP2C19, reducing conversion of clopidogrel to its active metabolite

Answer

Omeprazole induces CYP2C19, increasing clopidogrel activation

Answer

Clopidogrel increases omeprazole bioavailability through P-gp inhibition

Answer

Both drugs compete for the same platelet receptor binding sites

Question 4

Oral tetracyclines should not be taken with dairy products because calcium:

Accepted Answer

Forms insoluble chelate complexes with tetracyclines, reducing absorption

Answer

Increases gastric acid, inactivating the antibiotic

Answer

Competes with tetracyclines at intestinal efflux pumps

Answer

Accelerates renal excretion of tetracyclines

Question 5

Oral fluoroquinolones (e.g., ciprofloxacin) should be administered at least 2 hours before or 6 hours after antacids primarily because antacids:

Accepted Answer

Chelate fluoroquinolones via magnesium/aluminium ions, reducing absorption

Answer

Inhibit CYP3A4, increasing fluoroquinolone systemic exposure

Answer

Alkalinise the urine, promoting fluoroquinolone crystallisation in renal tubules

Answer

Accelerate gastric emptying, reducing fluoroquinolone contact time

Question 6

Co-prescribing methotrexate with NSAIDs increases the risk of methotrexate toxicity primarily because:

Accepted Answer

NSAIDs reduce renal clearance and tubular secretion of methotrexate, increasing plasma levels

Answer

NSAIDs induce DHFR, partially reversing methotrexate's mechanism of action

Answer

NSAIDs displace methotrexate from tissue binding sites, causing redistribution

Answer

NSAIDs inhibit thymidylate synthase, potentiating bone marrow toxicity

Question 7

Ciprofloxacin co-administered with theophylline is likely to cause which outcome?

Accepted Answer

Theophylline toxicity due to CYP1A2 inhibition by ciprofloxacin

Answer

Subtherapeutic theophylline levels due to CYP1A2 induction

Answer

Ciprofloxacin toxicity due to theophylline-mediated CYP3A4 inhibition

Answer

Additive nephrotoxicity requiring dose reductions of both agents