ANSWER: B

Rationale:

The 2022 ESC/ERS guidelines define PAH hemodynamically as a resting mean pulmonary arterial pressure (mPAP) greater than 20 mmHg measured by right heart catheterization (RHC), combined with a pulmonary arterial wedge pressure (PAWP) at or below 15 mmHg and a pulmonary vascular resistance (PVR) greater than 2 Wood units. The PAWP criterion establishes that the elevated pressure is precapillary (intrinsic to the pulmonary arterial bed) rather than transmitted backward from the left heart, and the PVR criterion confirms intrinsic pulmonary vascular disease.

• Option A: Option A is incorrect because the mPAP threshold of 25 mmHg is the prior (pre-2022) value; the 2022 guidelines lowered it to greater than 20 mmHg based on outcome data showing elevated mortality above this level, and a PAWP greater than 15 mmHg would indicate postcapillary disease rather than PAH.
• Option C: Option C is incorrect because a PVR below 2 Wood units does not meet the diagnostic threshold; PAH requires PVR greater than 2 Wood units to confirm intrinsic pulmonary vascular disease.
• Option D: Option D is incorrect because noninvasive echocardiographic estimates of systolic pulmonary pressure are used for screening only and cannot establish the diagnosis; RHC is mandatory to confirm PAH.
• Option E: Option E is incorrect because a PAWP greater than 15 mmHg defines postcapillary pulmonary hypertension (typically Group 2, from left heart disease), which is explicitly excluded from the PAH definition.

ANSWER: A

Rationale:

In PAH the three target pathways are dysregulated in opposite directions, which dictates whether therapy augments or blocks each one. The prostacyclin pathway is deficient: endothelial prostacyclin synthase expression is reduced, lowering prostacyclin (prostaglandin I2) production, so therapy supplies prostacyclin analogues or a prostacyclin receptor (IP receptor) agonist. The endothelin pathway is overactive: endothelin-1 (ET-1) production is increased, driving vasoconstriction and remodeling, so therapy blocks it with endothelin receptor antagonists. The nitric oxide (NO) pathway is deficient: reduced endothelial NO synthase activity lowers NO and downstream cyclic guanosine monophosphate (cGMP), so therapy amplifies residual signaling with phosphodiesterase-5 inhibitors or a soluble guanylate cyclase stimulator.

• Option B: Option B inverts every direction and would imply suppressing the prostacyclin and NO pathways, which is the opposite of correct management.
• Option C: Option C is incorrect because the pathways are not uniformly overactive; only endothelin is overactive, while prostacyclin and NO are deficient.
• Option D: Option D is incorrect because the endothelin pathway is overactive rather than deficient, and the NO pathway is deficient rather than overactive.
• Option E: Option E is incorrect because prostacyclin and NO are deficient rather than overactive, and endothelin is overactive rather than deficient.

ANSWER: C

Rationale:

Epoprostenol is synthetic prostacyclin (prostaglandin I2). It binds prostacyclin receptors (IP receptors) on pulmonary arterial smooth muscle cells, which are coupled through the Gs protein to adenylyl cyclase, raising intracellular cyclic adenosine monophosphate (cAMP); cAMP activates protein kinase A and produces vasodilation, antiproliferation, and inhibition of platelet aggregation. Its plasma half-life is extremely short, approximately 2 to 5 minutes at physiological pH, so it cannot be given orally or intermittently and must be delivered as a continuous IV infusion through a tunneled central catheter and ambulatory pump; abrupt interruption can precipitate fatal rebound PAH crisis.

• Option A: Option A is incorrect because epoprostenol does not block the ETA receptor (that is the mechanism of endothelin receptor antagonists) and is not an oral agent.
• Option B: Option B is incorrect because it does not inhibit PDE-5 and is not an inhaled drug.
• Option D: Option D is incorrect because it does not stimulate soluble guanylate cyclase (that describes riociguat) and is not given orally.
• Option E: Option E is incorrect because the half-life is minutes, not hours; the very short half-life is precisely why continuous infusion, not once-daily dosing, is mandatory.

ANSWER: D

Rationale:

Bosentan, a dual ETA/ETB endothelin receptor antagonist, carries a characteristic dose-dependent hepatotoxicity signal: elevations in the liver aminotransferases alanine aminotransferase (ALT) and aspartate aminotransferase (AST) occur in approximately 11 percent of treated patients. Because of this, monthly liver function test (LFT) monitoring is mandated throughout therapy, and this requirement is built into the Risk Evaluation and Mitigation Strategy (REMS) program along with monthly pregnancy testing.

• Option A: Option A is incorrect because hyperkalemia is not a recognized bosentan effect; potassium monitoring is associated with agents such as mineralocorticoid receptor antagonists, not ERAs.
• Option B: Option B is incorrect because bosentan does not cause a progressive restrictive ventilatory defect requiring scheduled pulmonary function testing.
• Option C: Option C is incorrect because predictable neutropenia is not the bosentan toxicity signal; the agent's hallmark laboratory concern is hepatic aminotransferase elevation, not the white cell line.
• Option E: Option E is incorrect because bosentan does not suppress the thyroid axis and does not require scheduled thyroid-stimulating hormone monitoring.

ANSWER: E

Rationale:

Phosphodiesterase-5 (PDE-5) inhibitors (sildenafil, tadalafil) and the soluble guanylate cyclase (sGC) stimulator riociguat both raise cyclic guanosine monophosphate (cGMP) but through complementary mechanisms: PDE-5 inhibitors block cGMP degradation while riociguat directly increases cGMP production. Used together they cause additive cGMP accumulation and severe symptomatic hypotension that has resulted in deaths in clinical trials, making this combination absolutely contraindicated; a washout (at least 24 hours for sildenafil, at least 48 hours for tadalafil) is required before switching to riociguat.

• Option A: Option A is incorrect because an ERA plus a PDE-5 inhibitor is the evidence-based upfront dual oral combination, not a contraindicated pairing.
• Option B: Option B is incorrect because an ERA plus an inhaled prostacyclin analogue is an accepted combination across different pathways, not a prohibited one.
• Option C: Option C is incorrect because a PDE-5 inhibitor plus an IP receptor agonist targets two distinct pathways (NO-cGMP and prostacyclin-cAMP) and is used in combination regimens.
• Option D: Option D is incorrect because calcium channel blockers are reserved for the small subset of acutely vasoreactive patients and are not the lethal-interaction pairing described; the defining contraindication is the PDE-5 inhibitor plus riociguat combination.

ANSWER: A

Rationale:

All three approved endothelin receptor antagonists (bosentan, ambrisentan, macitentan) are absolutely contraindicated in pregnancy because animal studies demonstrate severe teratogenicity at doses below the human therapeutic range. Women of childbearing potential must use two reliable forms of contraception throughout therapy and for one month after discontinuation, with monthly pregnancy testing mandated under a Risk Evaluation and Mitigation Strategy (REMS) program; a positive test requires immediate discontinuation and urgent obstetric consultation.

• Option B: Option B is incorrect and dangerously so, because ERAs are among the most clearly teratogenic of PAH drugs and are never considered safe in pregnancy.
• Option C: Option C is incorrect because the contraindication is absolute across all trimesters and during attempts to conceive, not limited to the third trimester.
• Option D: Option D is incorrect because the teratogenicity is intrinsic to the ERA itself and applies regardless of whether a PDE-5 inhibitor is co-prescribed.
• Option E: Option E is incorrect because a single barrier method is insufficient; two reliable forms of contraception plus monthly pregnancy testing are required.

ANSWER: B

Rationale:

Sildenafil is a selective phosphodiesterase-5 (PDE-5) inhibitor. PDE-5 is the dominant cyclic guanosine monophosphate (cGMP)-degrading enzyme in pulmonary arterial smooth muscle and is overexpressed in PAH; by blocking it, sildenafil prevents cGMP breakdown and amplifies the vasodilatory and antiproliferative signaling initiated by endothelial nitric oxide (NO). For PAH it is dosed at 20 mg three times daily, a regimen distinct from its erectile dysfunction dosing. It is absolutely contraindicated with nitrates of any formulation (including organic nitrates and NO donors) because the synergistic cGMP accumulation can produce severe, potentially fatal hypotension.

• Option A: Option A is incorrect because sildenafil inhibits PDE-5 rather than directly stimulating sGC (that is riociguat's mechanism), and nitrates are contraindicated, not safe.
• Option C: Option C is incorrect because sildenafil does not block the ETA receptor; that is the mechanism of selective ERAs such as ambrisentan.
• Option D: Option D is incorrect because sildenafil is not a prostacyclin analogue and does not act through cAMP, and its defining contraindication is with nitrates, not beta-blockers.
• Option E: Option E is incorrect because the nitrate interaction is a genuine and potentially lethal contraindication, not a negligible one.

ANSWER: C

Rationale:

Selexipag is an orally bioavailable, non-prostanoid selective prostacyclin receptor (IP receptor) agonist that is chemically distinct from the prostacyclin analogues. Its active metabolite binds the IP receptor with high selectivity and raises cyclic adenosine monophosphate (cAMP) in pulmonary arterial smooth muscle cells without the off-target prostanoid-receptor effects of nonselective analogues. The pivotal GRIPHON trial enrolled 1156 patients and demonstrated roughly a 40 percent reduction in the composite morbidity-mortality endpoint compared with placebo; selexipag is taken twice daily and titrated to the maximum tolerated dose.

• Option A: Option A is incorrect because selexipag is oral, not an inhaled analogue such as iloprost or inhaled treprostinil.
• Option B: Option B is incorrect because it is not an intravenous prostacyclin analogue; that description fits epoprostenol or IV treprostinil.
• Option D: Option D is incorrect because selexipag acts on the prostacyclin (IP receptor) pathway, not the endothelin pathway, and is not an ERA.
• Option E: Option E is incorrect because the sGC stimulator with a CTEPH indication is riociguat, not selexipag.

ANSWER: D

Rationale:

Macitentan is a dual ETA/ETB endothelin receptor antagonist distinguished by a highly lipophilic structure that confers enhanced tissue penetration and prolonged receptor occupancy relative to bosentan and ambrisentan. Its pivotal SERAPHIN trial enrolled 742 patients and was the first PAH trial powered for a morbidity-mortality composite endpoint; macitentan 10 mg reduced the composite of death or worsening PAH by about 45 percent versus placebo.

• Option A: Option A is incorrect because macitentan is a dual ETA/ETB antagonist rather than a selective ETA agent, and BREATHE-1 was the bosentan trial.
• Option B: Option B is incorrect because macitentan is highly lipophilic with strong tissue penetration, not hydrophilic with poor penetration, and SUPER-1 was the sildenafil trial.
• Option C: Option C is incorrect because macitentan is an ERA, not a prostacyclin analogue, and GRIPHON was the selexipag trial.
• Option E: Option E is incorrect because macitentan is not an sGC stimulator; PATENT-1 was the riociguat trial.

ANSWER: E

Rationale:

The endothelin receptor type B (ETB) has opposite effects depending on where it is expressed. On pulmonary arterial smooth muscle, ETB activation contributes to vasoconstriction, like the ETA receptor. On the endothelium, however, ETB mediates clearance of endothelin-1 (ET-1) from the circulation and promotes the release of the vasodilators prostacyclin and nitric oxide (NO). This dual role is the basis for the theoretical concern that blocking ETB (as dual antagonists do) might remove a beneficial endothelial action, though this has not translated into clinically meaningful outcome differences between selective and dual agents.

• Option A: Option A is incorrect because ETB receptors are present on both endothelium and smooth muscle and do not produce pure vasodilation everywhere.
• Option B: Option B is incorrect because the endothelial ETB receptor is central to ET-1 clearance, not merely to synthesis.
• Option C: Option C is incorrect because ETB receptors are functionally active and do influence pulmonary vascular tone and ET-1 handling.
• Option D: Option D inverts the locations: it is the smooth-muscle ETB that drives vasoconstriction and the endothelial ETB that clears ET-1, not the reverse.

ANSWER: A

Rationale:

Riociguat is a soluble guanylate cyclase (sGC) stimulator that acts through two complementary mechanisms. First, it sensitizes sGC to endogenous nitric oxide (NO), shifting the enzyme to its active conformation at lower NO concentrations than would otherwise be required. Second, it independently activates sGC at an NO-independent binding site, producing cGMP even when NO bioavailability is severely reduced, as occurs in advanced PAH endothelial dysfunction. This NO-independent action distinguishes it from PDE-5 inhibitors, which depend on residual NO-driven cGMP.

• Option B: Option B is incorrect because riociguat does not work by inhibiting PDE-5; it increases cGMP production rather than blocking its degradation.
• Option C: Option C is incorrect because riociguat does not block the ETA receptor; that is the mechanism of endothelin receptor antagonists.
• Option D: Option D is incorrect because riociguat acts on the NO-cGMP pathway through sGC, not on the prostacyclin-cAMP pathway through IP receptors.
• Option E: Option E is incorrect because riociguat does not supply exogenous NO; it acts on sGC itself, both NO-dependently and NO-independently.

ANSWER: B

Rationale:

Bosentan is a potent inducer of the cytochrome P450 (CYP) isoforms CYP3A4 and CYP2C9. CYP2C9 is the principal enzyme responsible for the metabolism of the pharmacologically active S-enantiomer of warfarin; induction of CYP2C9 accelerates warfarin clearance, reducing plasma warfarin concentrations and lowering the anticoagulant effect, reflected as a subtherapeutic INR. This interaction affects other CYP3A4 and CYP2C9 substrates as well, including cyclosporine, glyburide, and hormonal contraceptives, which is one reason two reliable forms of contraception rather than a hormonal method alone are required.

• Option A: Option A is incorrect because bosentan is an inducer rather than an inhibitor of CYP3A4, so it accelerates rather than reduces warfarin clearance, and the INR falls rather than rises.
• Option C: Option C is incorrect because protein displacement is not the clinically relevant mechanism and would transiently raise rather than lower the INR.
• Option D: Option D is incorrect because bosentan's drug interaction operates at the level of hepatic CYP induction, not renal tubular transport.
• Option E: Option E is incorrect because the CYP induction by bosentan is well-established and clinically significant, particularly for warfarin and cyclosporine.

ANSWER: C

Rationale:

Treprostinil is a chemically stable tricyclic benzindene prostacyclin analogue with a plasma half-life of approximately 4 hours, which is long enough to permit multiple administration routes: continuous subcutaneous (SC) infusion, continuous IV infusion, inhaled nebulization four times daily, and an extended-release oral tablet. The longer half-life also means interruption of SC or IV treprostinil is less immediately catastrophic than epoprostenol interruption, though rebound disease can still occur. The inhaled formulation was expanded beyond Group 1 PAH to include pulmonary hypertension associated with interstitial lung disease (PH-ILD, WHO Group 3b) based on the INCREASE trial.

• Option A: Option A is incorrect because treprostinil is available in four routes, not only IV.
• Option B: Option B is incorrect because the 2- to 5-minute half-life and mandatory continuous IV route describe epoprostenol, not treprostinil.
• Option D: Option D is incorrect because treprostinil is a genuine prostacyclin analogue, not a non-prostanoid IP agonist (that description fits selexipag), and it is approved beyond Group 1.
• Option E: Option E is incorrect because the six-to-nine-times-daily exclusive inhalation description fits iloprost, not treprostinil.

ANSWER: D

Rationale:

The AMBITION trial randomized 500 treatment-naive patients with WHO functional class (WHO-FC) II or III PAH to the combination of ambrisentan (an ERA) plus tadalafil (a PDE-5 inhibitor) versus either agent as monotherapy. The combination arm showed approximately a 50 percent reduction in the primary composite endpoint of time to first clinical failure event, which included death, hospitalization for PAH, disease progression, and unsatisfactory long-term clinical response, compared with the pooled monotherapy arms. This trial provided the core evidence base for the current standard of upfront dual oral combination in treatment-naive patients who are not at high risk.

• Option A: Option A is incorrect because AMBITION compared combination with monotherapy, not two monotherapies with each other.
• Option B: Option B is incorrect because AMBITION showed benefit for upfront oral dual combination, not upfront triple parenteral therapy (the latter is reserved for high-risk patients per guidelines).
• Option C: Option C is incorrect because the trial demonstrated the superiority of combination over PDE-5 inhibitor monotherapy, not the reverse.
• Option E: Option E is incorrect because AMBITION enrolled predominantly WHO-FC II and III patients, and its results inform the standard for those groups rather than being restricted to FC IV patients.

ANSWER: E

Rationale:

A positive acute vasodilator response is defined by three simultaneous criteria: a reduction in mean pulmonary arterial pressure (mPAP) of at least 10 mmHg, a resulting absolute mPAP below 40 mmHg, and maintenance or increase in cardiac output (CO). All three must be met; a fall in mPAP alone without the absolute threshold or without preserved CO does not qualify. Patients who meet these criteria, typically a small minority of those with idiopathic PAH who have WHO-FC I or II disease and no features of right heart failure, can be treated with high-dose calcium channel blockers (CCBs), specifically nifedipine, diltiazem, or amlodipine, as first-line monotherapy and have markedly better long-term prognosis than non-vasoreactive patients.

• Option A: Option A is incorrect because a 5-mmHg fall to any absolute value is insufficient and does not meet the accepted definition; the threshold is at least 10 mmHg to an absolute below 40 mmHg.
• Option B: Option B is incorrect because a rise in mPAP with a fall in CO indicates fixed, irreversible pulmonary vascular disease and worsening hemodynamics during the test, not a positive response.
• Option C: Option C is incorrect because systemic hypotension with unchanged mPAP describes an adverse systemic vasodilatory effect, not a pulmonary vasodilatory response.
• Option D: Option D is incorrect because a fall in CO is explicitly not part of a positive response definition; preserved or increased CO is required, not decreased CO.

ANSWER: A

Rationale:

PAH-specific vasodilator therapies (prostacyclin analogues, endothelin receptor antagonists, PDE-5 inhibitors, and sGC stimulators) are approved for WHO Group 1 PAH only. In Group 2 pulmonary hypertension (caused by left heart disease), elevated pulmonary pressures reflect transmitted backward pressure from the failing left heart rather than intrinsic pulmonary vascular disease; selectively vasodilating the pulmonary circulation in this setting can worsen left ventricular filling, precipitate pulmonary edema, and increase mortality. Right heart catheterization (RHC) is mandatory before initiating PAH-specific therapy to confirm precapillary hemodynamics (PAWP at or below 15 mmHg, PVR greater than 2 Wood units) and to exclude Group 2 or Group 3 disease.

• Option B: Option B is incorrect because these therapies are not approved across all five WHO groups and empirical initiation without catheterization is unsafe and outside guidelines.
• Option C: Option C is incorrect because Group 2 requires treatment of the underlying left heart disease, not PAH-specific vasodilator combination; using ERA and PDE-5 inhibitors in Group 2 has caused harm in clinical trials.
• Option D: Option D is incorrect because echocardiographic pulmonary pressure estimates are used only for screening and cannot confirm the hemodynamic group; RHC is mandatory for diagnosis.
• Option E: Option E is incorrect because lowering pulmonary pressures in Group 2 can worsen left ventricular preload and filling, producing clinical deterioration rather than benefit.

ANSWER: C

Rationale:

The 2022 ESC/ERS guidelines set low-risk status as the explicit treatment target in PAH, not merely clinical stability. The three-strata risk model categorizes patients using WHO-FC, 6MWD, NT-proBNP, echocardiographic RV function, and hemodynamics. A 6MWD of 310 meters, NT-proBNP above 300 ng/L, and persistent RV dilation place this patient at intermediate risk. The treatment principle is explicit: a patient who remains at intermediate risk after 3 to 6 months of dual oral combination has not achieved an adequate response, and persistent intermediate risk must be treated as a failure requiring escalation regardless of subjective symptom stability. Escalation options include adding an inhaled or subcutaneous prostacyclin agent or selexipag.

• Option A: Option A is incorrect because subjective stability is not the endpoint; the goal is objective low-risk status defined by the composite of functional, biomarker, and hemodynamic parameters.
• Option B: Option B is incorrect because high-dose CCBs are reserved for the small minority with a positive acute vasodilator response; they are not a retreatment option for a patient failing dual oral combination.
• Option D: Option D is incorrect because an ERA and an sGC stimulator (riociguat) can be combined safely; the prohibited combination is an sGC stimulator with a PDE-5 inhibitor.
• Option E: Option E is incorrect because anticoagulation is not an escalation step for PAH vasodilator therapy; the indication for warfarin in PAH is narrow and unrelated to hemodynamic escalation.

ANSWER: D

Rationale:

Riociguat is the only currently approved PAH vasodilatory agent with a formal indication for chronic thromboembolic pulmonary hypertension (CTEPH, WHO Group 4). This approval is based on the CHEST-1 trial (Chronic Thromboembolic Pulmonary Hypertension Soluble Guanylate Cyclase Stimulator Trial 1), which demonstrated significant improvements in 6-minute walk distance (6MWD) and pulmonary hemodynamics in patients with inoperable CTEPH or recurrent/persistent CTEPH after pulmonary endarterectomy. This CTEPH indication distinguishes riociguat from all other PAH vasodilators and reflects its mechanism: the ability to activate sGC even under low NO conditions, as occurs in the remodeled CTEPH vasculature.

• Option A: Option A is incorrect because BREATHE-1 was the bosentan PAH trial and bosentan does not have a formal CTEPH approval.
• Option B: Option B is incorrect because SUPER-1 studied sildenafil in WHO Group 1 PAH; sildenafil does not carry a formal CTEPH indication in its label.
• Option C: Option C is incorrect because the Barst 1996 trial established epoprostenol's survival benefit in severe PAH (Group 1), not in CTEPH, and epoprostenol does not carry a CTEPH indication.
• Option E: Option E is incorrect because GRIPHON enrolled WHO Group 1 PAH patients only; selexipag does not have a CTEPH approval.

ANSWER: E

Rationale:

Continuous IV epoprostenol has an extremely short plasma half-life of roughly 2 to 5 minutes. Abrupt interruption leads to immediate loss of pulmonary vasodilation and rapid rebound PAH crisis characterized by severe pulmonary vasoconstriction, right ventricular failure, systemic hypotension, and death, which can occur within minutes. The emergency response is immediate restoration of IV epoprostenol at the previous dose, or bridging with inhaled iloprost (a prostacyclin analogue) until IV access can be re-established. Patients and caregivers are trained to carry backup pump supplies and emergency medication precisely for this scenario, and emergency department staff must be educated about the lethality of delay.

• Option A: Option A is incorrect because oral bosentan has a slow onset over days to weeks and provides no acute hemodynamic rescue; it would be wholly inadequate in a PAH crisis from epoprostenol interruption.
• Option B: Option B is incorrect because oral sildenafil also has insufficient speed of action to reverse an acute rebound PAH crisis, and deferring catheterization is inappropriate in an acute hemodynamic collapse.
• Option C: Option C is incorrect because a 30-minute interruption in a patient with severe PAH is not inconsequential; it is an emergency requiring immediate action.
• Option D: Option D is incorrect because systemic anticoagulation does not address the acute vasoconstriction driving the crisis; restored prostacyclin delivery is the intervention.

ANSWER: B

Rationale:

Ambrisentan undergoes hepatic uptake via the organic anion transporting polypeptide OATP1B1. Cyclosporine is a potent inhibitor of OATP1B1; when co-administered, it blocks hepatic ambrisentan uptake and clearance, causing a substantial rise in ambrisentan plasma concentrations and increasing exposure-dependent adverse effects. This combination is considered clinically problematic and should generally be avoided. The bosentan-cyclosporine interaction operates through a completely different mechanism: bosentan is contraindicated with cyclosporine because bosentan induces CYP3A4, reducing cyclosporine levels, while cyclosporine concurrently raises bosentan levels, creating a bidirectional interaction.

• Option A: Option A is incorrect because cyclosporine is an OATP inhibitor, not a CYP3A4 inducer; the direction of the ambrisentan interaction is increased (not decreased) exposure.
• Option C: Option C is incorrect because ambrisentan acts on the endothelin receptor, not the calcineurin pathway, and does not antagonize cyclosporine's immunosuppressive mechanism.
• Option D: Option D is incorrect because ambrisentan does not significantly induce CYP enzymes, in contrast to bosentan; this is a distinguishing feature of ambrisentan.
• Option E: Option E is incorrect because the primary pharmacokinetic mechanism is hepatic transporter inhibition, not renal tubular competition.

ANSWER: A

Rationale:

Tadalafil is used in PAH at 40 mg once daily, studied in the PHIRST trial, which demonstrated a 33-meter improvement in 6-minute walk distance and reduced time to clinical worsening in treatment-naive patients. Sildenafil is used at 20 mg three times daily in PAH, a regimen from the SUPER-1 trial that is distinct from its higher and more flexible erectile dysfunction dosing. The once-daily tadalafil schedule provides a patient adherence advantage over the three-times-daily sildenafil regimen. Both agents share the class adverse effects of PDE-5 inhibition: headache, flushing, rhinitis, and visual color disturbance (the last resulting from inhibition of PDE-6, the isoform expressed in retinal photoreceptors, at higher drug concentrations). Both are absolutely contraindicated with nitrates.

• Option B: Option B is incorrect because the dosing frequencies are reversed: tadalafil is once daily and sildenafil is three times daily.
• Option C: Option C is incorrect because the doses and schedules differ and both carry clinically important class adverse effects.
• Option D: Option D is incorrect because it inverts the agents' dosing schedules; sildenafil is the three-times-daily agent and tadalafil is the once-daily agent.
• Option E: Option E is incorrect because both PDE-5 inhibitors are absolutely contraindicated with nitrates; the synergistic cGMP accumulation can cause severe, potentially fatal hypotension regardless of the mechanistic route to cGMP elevation.

ANSWER: C

Rationale:

This patient is clearly at high risk by every criterion of the 2022 ESC/ERS three-strata model: WHO-FC IV, 6MWD well below 165 meters, markedly elevated NT-proBNP, severely elevated right atrial pressure above 14 mmHg, and a low cardiac index below 2.0 L/min/m². The treatment algorithm for newly diagnosed high-risk or WHO-FC IV PAH patients who are vasoreactivity-negative is upfront triple combination: an endothelin receptor antagonist (ERA), a phosphodiesterase-5 (PDE-5) inhibitor, and a parenteral prostacyclin analogue, most commonly continuous IV epoprostenol given its survival-benefit evidence. Waiting to add parenteral prostacyclin only at deterioration would leave a high-risk patient on an inadequate regimen during the highest-risk period.

• Option A: Option A is incorrect because the patient is vasoreactivity-negative; high-dose CCBs are contraindicated in vasoreactivity-negative patients and would be dangerous in WHO-FC IV disease.
• Option B: Option B is incorrect because sequential monotherapy is the approach the AMBITION and contemporary trials have supplanted; high-risk patients require the most intensive upfront regimen, not a staged single-agent approach.
• Option D: Option D is incorrect because dual oral combination without parenteral prostacyclin is the recommended starting point for low- or intermediate-risk patients (WHO-FC II or III), not for a WHO-FC IV high-risk patient with this hemodynamic profile.
• Option E: Option E is incorrect because riociguat monotherapy is not the guideline-recommended initial regimen for any risk stratum; riociguat or a PDE-5 inhibitor would be one component of a combination regimen, never the sole agent in high-risk disease.