PSA Drug Interactions 4

Question 1

A patient stabilized on lithium takes ibuprofen for 5 days. Lithium levels rise into the toxic range. What is the mechanism?

Accepted Answer

Ibuprofen reduces renal prostaglandin synthesis, impairing lithium clearance and raising serum levels

Answer

NSAIDs suppress prostaglandin-mediated afferent arteriolar dilation in the kidney, reducing GFR and the renal clearance of lithium, which has no hepatic metabolism.

Question 2

A patient on an irreversible MAOI consumes aged cheese and cured meats. What dangerous reaction is precipitated?

Accepted Answer

Hypertensive crisis from tyramine-induced massive catecholamine release

Answer

MAO-A in the intestinal wall normally metabolizes dietary tyramine; when inhibited, tyramine enters systemic circulation and triggers massive norepinephrine release from sympathetic nerve terminals, causing hypertensive crisis.

Question 3

Sildenafil is absolutely contraindicated with organic nitrates because this combination causes:

Accepted Answer

Synergistic elevation of cGMP via combined NO and PDE5 pathways causing severe hypotension

Answer

Nitrates release nitric oxide (stimulating guanylyl cyclase) and sildenafil blocks cGMP degradation (inhibiting PDE5); together they cause profound cGMP-mediated smooth muscle relaxation and potentially fatal hypotension.

Question 4

IV iodinated contrast is administered to a patient on metformin. The primary pharmacological risk is:

Accepted Answer

Contrast-induced nephropathy impairing metformin renal elimination, risking lactic acidosis

Answer

Iodinated contrast can cause acute tubular injury; if metformin clearance falls due to reduced GFR, it accumulates and inhibits hepatic mitochondrial respiration, promoting lactic acidosis.

Question 5

A patient on warfarin is prescribed metronidazole for a dental infection. The INR rises from 2.5 to 7.0. What is the primary mechanism?

Accepted Answer

Metronidazole inhibits CYP2C9, the main enzyme metabolizing the more potent S-warfarin enantiomer

Answer

Metronidazole is a potent CYP2C9 inhibitor; since S-warfarin (the more pharmacologically active enantiomer) is primarily metabolized by CYP2C9, its levels rise substantially, dramatically increasing the INR.

(PSA) Prescribing Safety Assessment Practice Test

(PSA) Prescribing Safety Assessment Practice Test

PSA Drug Interactions 4