ANSWER: B

Rationale:

Dofetilide is eliminated almost entirely by renal excretion, and the primary mechanism of renal tubular secretion is organic cation transporter 2 (OCT2). Trimethoprim is a potent inhibitor of OCT2 and is listed as a contraindicated combination in the dofetilide prescribing label. When OCT2 is inhibited, dofetilide cannot be secreted efficiently into the tubular lumen, renal elimination falls, plasma dofetilide concentrations rise, IKr blockade increases, and the QTc prolongs to dangerous levels. The dofetilide label specifically lists trimethoprim as contraindicated, along with verapamil, cimetidine, ketoconazole, and other OCT2 or renal secretion inhibitors. This interaction illustrates why the prescribing environment for dofetilide requires careful review of all co-medications at every encounter, not just at initiation. In this patient, the QTc has risen from 460 ms to 540 ms and polymorphic ventricular tachycardia has occurred; dofetilide must be discontinued and trimethoprim stopped, with alternative antibiotics selected for the urinary tract infection.

• Option A: Option A is incorrect: dofetilide is not primarily metabolized by CYP3A4; its elimination is predominantly renal through OCT2-mediated secretion, and CYP3A4 inhibition is not the mechanism of this interaction; ketoconazole, a CYP3A4 inhibitor, is also contraindicated with dofetilide, but through a different mechanism involving both CYP3A4 and renal transport inhibition.
• Option C: Option C is incorrect: protein binding displacement is generally not a clinically significant mechanism for drug interactions with dofetilide, and trimethoprim does not act through this mechanism; dofetilide is approximately 60 to 70 percent protein bound, but displacement alone is rarely the cause of meaningful toxicity.
• Option D: Option D is incorrect: while trimethoprim may have some weak IKr blocking properties, the primary and clinically dominant interaction mechanism is pharmacokinetic via OCT2 inhibition, not pharmacodynamic IKr synergy; the dofetilide label contraindication is based on the pharmacokinetic interaction.
• Option E: Option E is incorrect: urinary pH manipulation through alkalinization is not the mechanism of the trimethoprim-dofetilide interaction; OCT2-mediated secretion, not passive pH-dependent reabsorption, governs dofetilide renal elimination.

ANSWER: D

Rationale:

Amiodarone is a potent inhibitor of CYP2C9, the cytochrome P450 isoform responsible for the metabolism of S-warfarin, the more pharmacologically active enantiomer of racemic warfarin. When amiodarone inhibits CYP2C9, S-warfarin clearance is reduced, plasma S-warfarin concentrations rise, and the anticoagulant effect increases substantially. This interaction is further complicated by amiodarone's extremely long half-life (weeks to months due to extensive tissue accumulation), which means the interaction develops slowly over weeks, may peak after six to eight weeks of concurrent therapy, and persists long after amiodarone is discontinued. When amiodarone is added to a stable warfarin regimen, warfarin dose reductions of 30 to 50 percent are typically required to maintain the INR within the therapeutic range, and close INR monitoring is mandatory during both initiation and any subsequent amiodarone dose changes or discontinuation. In this patient, the INR of 4.8 with minor bleeding requires prompt warfarin dose reduction and more frequent INR monitoring until stability is re-established.

• Option A: Option A is incorrect: amiodarone is a CYP2C9 inhibitor, not an inducer; enzyme induction would reduce warfarin effect and lower the INR, the opposite of what is observed in this patient.
• Option B: Option B is incorrect: albumin displacement can transiently raise free drug concentrations but is not a clinically significant sustained mechanism for the amiodarone-warfarin interaction; total warfarin concentrations rise due to reduced metabolic clearance, not displacement.
• Option C: Option C is incorrect: P-glycoprotein inhibition by amiodarone in the gut wall is not the primary mechanism of the warfarin interaction; warfarin absorption is not substantially P-glycoprotein dependent, and the interaction is metabolic rather than absorptive.
• Option E: Option E is incorrect: amiodarone does not inhibit vitamin K epoxide reductase; warfarin itself inhibits this enzyme, and amiodarone's effect on anticoagulation is through raising warfarin plasma concentrations via CYP2C9 inhibition, not through additive pharmacodynamic anticoagulation.

ANSWER: A

Rationale:

Quinidine, unlike other Class Ia agents, has clinically significant antimuscarinic (anticholinergic) properties. By blocking muscarinic M2 receptors on sinoatrial and atrioventricular nodal cells, quinidine removes the normally present vagal tone that acts as a brake on both the sinus rate and AV nodal conduction velocity. The result is sinus tachycardia, which explains this patient's presentation. The AV nodal effect carries additional clinical significance in patients with atrial flutter: quinidine slows the atrial flutter rate (through its Class Ia sodium channel blocking effect), and simultaneously enhances AV nodal conduction through M2 blockade. This combination can convert a 2:1 flutter pattern (ventricular rate 150 beats per minute) to 1:1 conduction at a slower flutter rate, paradoxically accelerating the ventricular rate, a recognized and dangerous form of proarrhythmia. For this reason, quinidine given for atrial flutter should always be preceded or accompanied by an AV nodal blocking agent. Procainamide and disopyramide, the other Class Ia agents, also have antimuscarinic properties but generally less pronounced than quinidine.

• Option B: Option B is incorrect: quinidine prolongs, not shortens, action potential duration in sinus node cells; prolonged APD would be expected to slow, not accelerate, pacemaker firing.
• Option C: Option C is incorrect: quinidine does not inhibit the sodium-potassium ATPase; this mechanism describes digoxin toxicity; quinidine's electrophysiological actions are mediated through ion channel blockade, not pump inhibition.
• Option D: Option D is incorrect: while quinidine does have some alpha-adrenergic blocking properties that can cause vasodilation and reflex tachycardia, this is a secondary mechanism; the primary explanation for sinus tachycardia in patients on quinidine is direct antimuscarinic effect at the SA node, not baroreceptor-mediated reflex.
• Option E: Option E is incorrect: quinidine blocks potassium channels in a manner that prolongs the action potential, but this does not shorten the sinus cycle length; blocking IKr prolongs repolarization in working myocardium and is the basis of QT prolongation, not sinus tachycardia.

ANSWER: E

Rationale:

Reverse use-dependence is a pharmacodynamic property of Class III antiarrhythmic agents that is distinct from, and opposite to, the use-dependence seen with Class I agents. In reverse use-dependence, the drug's primary effect, which for Class III agents is action potential duration (APD) prolongation through IKr blockade, is most pronounced at slow heart rates and diminishes at faster rates. At slower heart rates, each diastolic interval is longer, the IKr channel dwells in a drug-accessible conformation for a longer period per cycle, and cumulative channel block is greater. This produces more pronounced APD prolongation and QT prolongation at slow rates. Conversely, at faster rates, shorter diastolic intervals leave less time for drug-channel interaction and the APD-prolonging effect is attenuated. The clinical consequence is that QT prolongation and torsades de pointes risk with Class III agents are paradoxically greatest during bradycardia, such as nocturnal resting rates or rate-controlled atrial fibrillation with slow ventricular response. This is the opposite of the therapeutic goal: ideally, an antiarrhythmic effect should be present when tachyarrhythmias occur (fast rates) and minimal when the rhythm is slow. Reverse use-dependence thus represents a pharmacodynamic limitation of Class III agents and explains why torsades de pointes with sotalol, dofetilide, and other Class III drugs often occurs during bradycardia or at the onset of a pause rather than during rapid tachycardia.

• Option A: Option A is incorrect: this description inverts the correct phenomenon; use-dependence applies to Class I agents and describes accumulating block at faster rates, not to Class III potassium channel blockers.
• Option B: Option B is incorrect: IKACh channels are primarily expressed in nodal tissue rather than working ventricular myocytes, and nocturnal parasympathetic augmentation does not produce meaningful additive potassium channel blockade in ventricular tissue in a clinically significant manner; reverse use-dependence is the correct explanation.
• Option C: Option C is incorrect: sotalol is not metabolized by CYP2D6 to a clinically significant degree; it is primarily renally eliminated unchanged; circadian variation in CYP2D6 activity is not the mechanism of this observation.
• Option D: Option D is incorrect: while physiological rate-adaptation does affect the action potential, the QTc correction formulas (Bazett, Fridericia) are validated to account for rate-related changes in QT; genuinely prolonged QTc values at slow rates reflect drug effect, not correction formula artifact.

ANSWER: C

Rationale:

Lidocaine systemic toxicity follows a predictable concentration-dependent sequence that provides a clinically useful warning hierarchy. CNS symptoms appear first and at lower plasma concentrations than cardiac toxicity, because neural tissue is more sensitive to sodium channel blockade than cardiac tissue at equivalent drug exposures. At mildly elevated plasma lidocaine concentrations (typically in the range of 3 to 6 mcg/mL), patients develop circumoral numbness, tinnitus, lightheadedness, and slurred speech. At higher concentrations (approximately 5 to 9 mcg/mL), muscle twitching and seizures occur. Cardiac toxicity manifesting as QRS widening, conduction block, and ventricular arrhythmias generally requires plasma concentrations of 8 to 12 mcg/mL or higher, roughly two to four times the concentration producing initial CNS symptoms. This concentration window means that CNS manifestations serve as an early warning of lidocaine toxicity, and recognition of circumoral tingling or tinnitus in a patient receiving lidocaine should prompt immediate dose reduction or cessation before cardiac toxicity supervenes. In this patient, the normal QRS and heart rate confirm that cardiac sodium channels have not yet been affected to a clinically significant degree, consistent with CNS symptoms at concentrations that have not yet reached the cardiac toxicity threshold.

• Option A: Option A is incorrect: while lidocaine does have differential affinity for sodium channel subtypes, the primary reason CNS toxicity precedes cardiac toxicity is the differential sensitivity of neural versus cardiac tissue at equivalent plasma concentrations, not absolute subtype selectivity.
• Option B: Option B is incorrect: lidocaine does have high lipid solubility and excellent CNS penetration, but the explanation for the toxicity sequence is tissue sensitivity, not preferential CNS accumulation that leaves plasma concentrations below the cardiac threshold; at the plasma concentrations producing seizures, lidocaine is present in cardiac tissue as well.
• Option D: Option D is incorrect: while lidocaine does have some inhibitory effects on neuronal sodium channels that disinhibit GABAergic pathways and can produce excitatory CNS symptoms, the mechanism of lidocaine CNS toxicity is primarily sodium channel dependent, not GABA receptor antagonism; this option also incorrectly implies a pharmacodynamic dissociation between CNS and cardiac effects.
• Option E: Option E is incorrect: lidocaine is metabolized by hepatic amidases (not esterases) to monoethylglycinexylidide (MEGX); while MEGX is an active metabolite with CNS effects, the clinical toxicity sequence described is primarily attributable to the parent compound lidocaine at supratherapeutic plasma concentrations, not to selective MEGX accumulation.

ANSWER: B

Rationale:

Procainamide-induced lupus-like syndrome (DILS) is among the most common and clinically significant adverse effects of long-term procainamide therapy. The mechanism involves conversion of procainamide to reactive metabolites, particularly through N-oxidation pathways, that alter nuclear protein antigenicity and trigger an autoimmune response. Antinuclear antibody positivity develops in 50 to 80 percent of patients on chronic procainamide therapy, though only 20 to 30 percent develop clinical symptoms. The clinical syndrome closely resembles idiopathic systemic lupus erythematosus, with arthralgia, pleuritis, pericarditis, and rash, but differs in that anti-double-stranded DNA antibodies are typically absent (ANA positive, anti-dsDNA negative distinguishes DILS from idiopathic SLE). Slow acetylators are at substantially greater risk because they accumulate higher concentrations of the parent procainamide compound or its hydroxylamine metabolites, which are believed to drive the autoimmune process; fast acetylators convert procainamide more rapidly to NAPA (N-acetylprocainamide), which has less immunogenic potential. The syndrome is generally reversible after procainamide discontinuation, though ANA titers may persist for months.

• Option A: Option A is incorrect: NAPA nephrotoxicity is not the mechanism of procainamide-induced lupus; NAPA accumulation in renal impairment is a concern for drug accumulation and proarrhythmia, not for autoimmune induction through complement activation.
• Option C: Option C is incorrect: anti-dsDNA antibodies are typically absent in procainamide-induced lupus, which is one of the key distinguishing features from idiopathic SLE; the claim that all patients develop measurable anti-dsDNA antibodies is factually wrong.
• Option D: Option D is incorrect: sodium channel blockade in T-cells is not the mechanism of procainamide immunogenicity; fast acetylators, not slow acetylators, are at lower risk because rapid conversion to NAPA reduces exposure to the more immunogenic procainamide metabolites.
• Option E: Option E is incorrect: procainamide-induced lupus does not occur uniformly in all patients within 12 months, does not result from direct C1q activation, and is generally reversible with drug discontinuation rather than requiring permanent immunosuppression.

ANSWER: D

Rationale:

Amiodarone contains approximately 37 percent iodine by weight, and a standard 200 mg daily dose releases approximately 7 to 9 mg of free iodine daily, representing up to 50 times the normal dietary iodine intake. This iodine excess can cause thyroid dysfunction through mechanisms that depend on underlying thyroid anatomy and autoimmune status. Type 1 amiodarone-induced thyrotoxicosis (AIT-1) occurs in glands with pre-existing functional abnormalities, such as multinodular goiter or subclinical Graves disease, where excess iodine fuels autonomous thyroid hormone synthesis (the Jod-Basedow phenomenon). Type 2 AIT occurs in anatomically normal glands, as in this patient, where amiodarone or its metabolites cause a direct destructive thyroiditis, damaging follicular cells and releasing preformed thyroid hormone into the circulation. The clinical presentation of both subtypes is similar (suppressed TSH, elevated free T4, symptoms of thyrotoxicosis), but the distinction matters for treatment: AIT-1 is treated with thionamides (methimazole or propylthiouracil) to block new hormone synthesis, while AIT-2 is treated with corticosteroids to suppress the inflammatory thyroid destruction. Color-flow Doppler thyroid ultrasonography can assist in distinguishing the two types based on thyroid vascularity. Amiodarone also causes hypothyroidism (more common than hyperthyroidism in iodine-sufficient populations) through the Wolff-Chaikoff effect, where the acute iodine load inhibits thyroid hormone synthesis.

• Option A: Option A is incorrect: amiodarone does not stimulate TSH receptors; the mechanism of thyroid dysfunction is iodine-mediated or destructive, not receptor agonism.
• Option B: Option B is incorrect: amiodarone does inhibit type 1 deiodinase and does cause some decrease in T3 with compensatory T4 elevation as a baseline pharmacological effect, but this physiological adaptation produces mildly elevated T4 and normal or mildly elevated TSH, not the markedly elevated free T4 and undetectable TSH seen in clinical thyrotoxicosis; deiodinase inhibition alone does not explain the clinical syndrome of amiodarone-induced thyrotoxicosis.
• Option C: Option C is incorrect: the mechanism of AIT-2 does involve thyroid follicle damage, but it is not through molecular mimicry or cytotoxic T-lymphocyte destruction in the classical autoimmune sense; the destructive process appears to be a direct toxic effect of amiodarone or its metabolites on follicular cells, not immune-mediated in the way this option describes.
• Option E: Option E is incorrect: AIT-2 specifically occurs in patients with anatomically normal thyroid glands who do not have pre-existing Graves disease; the exclusive Graves predisposition claim describes only one subtype (AIT-1) and incorrectly excludes the major category of amiodarone thyrotoxicosis in this patient's presentation.

ANSWER: A

Rationale:

The amiodarone-digoxin interaction is a well-established pharmacokinetic interaction mediated primarily by P-glycoprotein (P-gp) inhibition. P-gp is an efflux transporter expressed in intestinal epithelium, renal proximal tubular cells, and hepatocyte canalicular membranes, among other sites. Digoxin depends on P-gp for both its intestinal efflux (which limits oral bioavailability) and its renal tubular secretion (which is a major elimination pathway). When amiodarone inhibits P-gp, both mechanisms are impaired: intestinal P-gp inhibition increases digoxin absorption and bioavailability, while renal tubular P-gp inhibition reduces digoxin elimination, collectively producing a rise in steady-state serum digoxin concentrations. In practice, serum digoxin concentrations typically double when amiodarone is added, and the interaction develops gradually over weeks as amiodarone accumulates in tissues. The recommended management is an empiric 50 percent reduction in the digoxin dose when amiodarone is initiated, with subsequent titration guided by serum digoxin levels and clinical assessment. In this patient, the digoxin level of 1.9 ng/mL combined with symptoms of toxicity (nausea, visual changes) requires prompt digoxin dose reduction or temporary discontinuation.

• Option B: Option B is incorrect: amiodarone is a P-gp inhibitor, not a CYP3A4 inducer; CYP3A4 induction would accelerate drug metabolism and lower, not raise, serum concentrations; digoxin has minimal CYP3A4 metabolism.
• Option C: Option C is incorrect: amiodarone does not displace digoxin from Na+/K+-ATPase binding sites; this concept is pharmacologically incorrect and does not account for the observed pharmacokinetic change in steady-state drug levels; the interaction is a genuine pharmacokinetic change in clearance and bioavailability.
• Option D: Option D is incorrect: amiodarone does not cause clinically significant renal vasoconstriction through alpha-adrenergic blockade to a degree that meaningfully reduces glomerular filtration; the digoxin interaction is transporter-mediated, not hemodynamically mediated.
• Option E: Option E is incorrect: digoxin undergoes minimal hepatic metabolism; it is primarily renally eliminated and is not a CYP2C9 substrate; CYP2C9 inhibition by amiodarone is the basis of the warfarin interaction, not the digoxin interaction.

ANSWER: E

Rationale:

This clinical scenario illustrates a well-recognized and dangerous form of proarrhythmia with quinidine in atrial flutter that is entirely mechanistic and predictable. Two pharmacological properties of quinidine act simultaneously to produce this outcome. First, quinidine's Class Ia sodium channel blockade slows atrial conduction velocity in a use-dependent manner, reducing the flutter rate from 300 to 220 beats per minute. Second, quinidine's antimuscarinic (vagolytic) properties remove physiological vagal tone from the AV node, enhancing AV nodal conduction velocity and reducing AV nodal refractoriness. At the original flutter rate of 300 beats per minute, the AV node filtered the impulses with 2:1 block, producing a ventricular rate of 150 beats per minute. At the slowed flutter rate of 220 beats per minute, each flutter impulse arrives at a longer interval, and the enhanced AV nodal conduction (from vagolytic effect) allows each impulse to traverse the AV node before the next arrives, producing 1:1 conduction at 220 beats per minute. A ventricular rate of 210 beats per minute in this context is hemodynamically devastating, particularly in a patient with any degree of diastolic dysfunction or volume dependence. This outcome is precisely why quinidine (and other Class Ia agents) must always be co-administered with an AV nodal blocking agent (beta-blocker, diltiazem, verapamil, or digoxin) before initiating pharmacological flutter rate slowing.

• Option A: Option A is incorrect: quinidine's flutter rate slowing is through Class I sodium channel block, not Class III; potassium channel blocking (APD prolongation) is part of quinidine's Class Ia profile, but the mechanism of atrial rate slowing is sodium channel dependent conduction slowing, not cycle length stabilization.
• Option B: Option B is incorrect: while quinidine does prolong the QT interval and can cause TdP, the described scenario of a slowed atrial rate to 220 beats per minute followed by 1:1 ventricular conduction at 210 beats per minute is the classic quinidine syncope mechanism from 1:1 flutter conduction, not polymorphic VT; the two mechanisms are distinct and require different management.
• Option C: Option C is incorrect: the described mechanism would produce an irregular ventricular response, but the telemetry shows a regular 1:1 pattern, which is inconsistent with AF; conversion to AF would not produce a regular 1:1 pattern.
• Option D: Option D is incorrect: quinidine does not characteristically cause SA node arrest; sinus node arrest is a recognized but uncommon complication, and the resulting escape rhythm at 210 beats per minute is mechanistically implausible for a junctional escape, which would be expected at 40 to 60 beats per minute.

ANSWER: C

Rationale:

Ibutilide is an intravenous Class III antiarrhythmic agent with a unique mechanism that distinguishes it from other Class III drugs. In addition to IKr blockade (the defining Class III mechanism shared with sotalol and dofetilide), ibutilide activates a slow inward plateau sodium current, which further prolongs atrial action potential duration and refractoriness. This dual mechanism makes it particularly effective for cardioversion of atrial flutter, with conversion rates in the range of 60 to 70 percent for flutter and somewhat lower rates for atrial fibrillation. Ibutilide is administered as an intravenous infusion (1 mg over 10 minutes, repeated once if needed) and acts relatively rapidly. The primary safety concern is QT prolongation leading to torsades de pointes, which occurs in approximately 4 to 8 percent of patients. Consequently, the FDA mandates post-infusion monitoring with continuous ECG telemetry for a minimum of four hours after administration (or until the QTc returns to baseline), with resuscitative equipment immediately available. Ibutilide should not be used in patients with hypokalemia, hypomagnesemia, prolonged baseline QTc, or significant structural heart disease.

• Option A: Option A is incorrect: ibutilide is a Class III agent, not a Class Ic agent; 30 minutes of monitoring is grossly insufficient given the post-infusion TdP risk that can occur hours after administration.
• Option B: Option B is incorrect: ibutilide has no beta-adrenergic blocking activity; it is a pure antiarrhythmic agent acting through ion channel effects, not through adrenergic receptor blockade.
• Option D: Option D is incorrect: ibutilide has no adenosine receptor agonist activity and does not produce AV nodal block through parasympathetic mechanisms; its mechanism is entirely through direct ion channel effects prolonging atrial refractoriness.
• Option E: Option E is incorrect: ibutilide is a Class III agent, not a Class Ia agent; it does not have the same pharmacological profile or clinical indications as procainamide; its monitoring requirements are based on post-infusion TdP risk specific to intravenous Class III use.

ANSWER: B

Rationale:

Reverse use-dependence is a fundamental pharmacodynamic limitation of Class III antiarrhythmic agents that is particularly relevant for sotalol. Unlike Class I agents, which produce greater channel block at faster rates (use-dependence), Class III agents exhibit the opposite behavior: their action potential duration prolonging effect is most pronounced at slow heart rates and diminishes as rate increases. For sotalol, IKr block during the prolonged diastolic interval of bradycardia allows greater drug-channel interaction per cycle, producing more pronounced APD extension and QT prolongation. Conversely, at faster rates, shorter diastolic intervals reduce drug-channel interaction time per cycle, and the QT-prolonging effect is attenuated. The clinical consequence is that TdP with sotalol is paradoxically most likely to occur during bradycardia, rate-controlled AF with slow ventricular response, or following a pause rather than during rapid tachycardia. This creates a therapeutic paradox: the conditions under which sotalol is most effective at maintaining sinus rhythm (adequate rate control, low baseline heart rate) are also the conditions under which its proarrhythmic risk is highest. Monitoring sotalol-treated patients for excessive QTc prolongation at slow heart rates, particularly nocturnal rates, is therefore clinically essential.

• Option A: Option A is incorrect: this describes use-dependence, which is the property of Class I agents; sotalol's IKr blocking effect does not accumulate during tachycardia in this manner; the reverse is true.
• Option C: Option C is incorrect: sotalol's Class II beta-blocking effect does attenuate heart rate increases during activity, but it does not eliminate the reverse use-dependence of the Class III IKr block; the two pharmacological mechanisms operate through different channels and do not cancel each other's rate-dependent behavior in this manner.
• Option D: Option D is incorrect: reverse use-dependence does reduce (not eliminate) the QT-prolonging effect at faster rates, but the drug effect does not disappear entirely at heart rates above 100 beats per minute; QT prolongation is attenuated, not abolished, at faster rates.
• Option E: Option E is incorrect: reverse use-dependence is an intrinsic property of the drug-channel interaction kinetics and applies equally to both oral and intravenous sotalol; plasma concentration does not override the kinetic properties of drug-channel binding and dissociation.

ANSWER: D

Rationale:

Disopyramide is distinguished from the other Class Ia agents (quinidine and procainamide) by its pronounced negative inotropic effect on ventricular myocardium. This property makes it the most hazardous Class Ia agent in patients with impaired systolic function and can precipitate acute decompensated heart failure even in patients with mildly to moderately reduced ejection fraction. Among Class Ia agents, disopyramide has the greatest negative inotropic potency, attributed in part to its direct calcium channel-blocking properties in addition to sodium channel blockade. An LVEF of 45% with left ventricular hypertrophy and diastolic dysfunction in this patient places him at meaningful risk for clinical decompensation if disopyramide impairs systolic function further. In contrast, disopyramide is specifically used in obstructive hypertrophic cardiomyopathy precisely because its negative inotropic effect reduces the dynamic left ventricular outflow tract gradient, a niche indication where the property that is usually an adverse effect becomes therapeutically beneficial.

• Option A: Option A is incorrect: disopyramide does not cause peripheral vasodilation or reflex tachycardia; it has antimuscarinic properties (which can produce sinus tachycardia) and negative inotropic properties, but not alpha-adrenergic blocking vasodilation.
• Option B: Option B is incorrect: while disopyramide does undergo partial renal elimination and QT prolongation is a concern with all Class Ia agents, the most clinically significant adverse effect that is particular to disopyramide and relevant in a patient with borderline systolic function is negative inotropy, not QT prolongation.
• Option C: Option C is incorrect: disopyramide's antimuscarinic properties do cause urinary retention and constipation, and this is a genuine clinical concern in older men with benign prostatic hyperplasia; however, in the context of the question's framing asking about risk in a patient with reduced systolic function, negative inotropy is the dominant concern.
• Option E: Option E is incorrect: disopyramide does not inhibit aldosterone receptors; it has no recognized pharmacological effect on mineralocorticoid receptors, and this mechanism is pharmacologically fabricated.

ANSWER: A

Rationale:

Long QT syndrome type 3 is caused by gain-of-function mutations in SCN5A, the gene encoding the cardiac sodium channel Nav1.5. Normal sodium channels inactivate rapidly after opening during phase 0, but LQT3-associated mutations impair inactivation, producing a persistent late inward sodium current (INa,late) that continues to depolarize the cell throughout the plateau phase (phase 2) and prolongs action potential duration. Mexiletine is a Class Ib sodium channel blocker with rapid binding and unbinding kinetics that preferentially targets sodium channels in their inactivated state, the state in which the mutant LQT3 channels are persistently conducting. By blocking INa,late, mexiletine reduces the abnormal depolarizing current during the plateau phase, shortens action potential duration, and reduces the QTc interval in LQT3 patients. This is a mechanistically targeted therapy because it directly addresses the pathological current responsible for QT prolongation in this specific subtype. In LQT1 and LQT2, QT prolongation results from loss-of-function mutations in potassium channels (KCNQ1, encoding the slow delayed rectifier channel IKs, and KCNH2, encoding the rapid delayed rectifier channel IKr, respectively) that reduce repolarizing current; mexiletine's sodium channel blocking effect has no mechanistic rationale in these subtypes and does not shorten the QTc effectively in them. Clinical data support QTc shortening with mexiletine in LQT3, and it can be combined with a beta-blocker for additive benefit.

• Option B: Option B is incorrect: mexiletine is a Class Ib sodium channel blocker; it does not block IKr potassium channels; IKr blockade would prolong, not shorten, action potential duration and would worsen rather than improve QT prolongation in any LQT subtype.
• Option C: Option C is incorrect: mexiletine does have good oral bioavailability and serves as the oral equivalent of intravenous lidocaine for ventricular arrhythmias, but the claim that it has no mechanistic specificity for LQT3 is incorrect; its mechanism of blocking INa,late is directly and specifically relevant to the LQT3 substrate.
• Option D: Option D is incorrect: mexiletine's primary mechanism is sodium channel blockade, not L-type calcium channel blockade; calcium channel blockade is the mechanism of verapamil and diltiazem (Class IV agents); mexiletine does not exert clinically significant calcium channel effects.
• Option E: Option E is incorrect: mexiletine has no aldosterone receptor antagonist properties; this mechanism describes spironolactone, and reducing systemic sodium reabsorption does not affect the intracellular sodium concentration in ventricular myocytes through any established pathway.