These drugs work as inodilators in heart failure. They boost the heart’s pumping strength (positive inotropy) and also widen the peripheral vessels (vasodilation), which lowers the afterload the heart has to push against. Dobutamine mainly stimulates beta-1 receptors to increase contractility, with some beta-2–mediated vasodilation that lowers systemic vascular resistance. Milrinone inhibits PDE-3, raising cAMP in both heart muscle and vascular smooth muscle, which directly increases contractility and causes vasodilation. The net result is higher cardiac output with reduced systemic vascular resistance, fitting the description of increasing contractility and decreasing afterload. Other options describe either reduced contractility, increased afterload, or no change in vascular resistance, which don’t match how these drugs work. These agents are typically used short-term in acute settings, with attention to potential tachycardia or hypotension.

Rupture of a vulnerable plaque with subsequent thrombosis is the event that directly leads to artery occlusion at this stage. When an atherosclerotic plaque becomes unstable, its fibrous cap thins and the underlying lipid-rich core is exposed to flowing blood. Inflammatory processes and enzymes weaken the cap, and a rupture allows platelets to adhere and the coagulation cascade to activate, forming a thrombus. This clot can rapidly grow and block the vessel, cutting off blood flow and causing acute ischemia or infarction. Calcification, while a marker of advanced disease, tends to stiffen plaques and is not the immediate cause of occlusion. Endothelial dilation is not the characteristic event driving occlusion at this stage, and systemic hypertension is a risk factor that accelerates atherosclerosis but does not itself cause the acute occlusive event.

Phase 0 in the SA node is driven by calcium entry through L-type calcium channels. Pacemaker cells rely on Ca2+ for the rapid depolarization because they lack the fast Na+ current that drives phase 0 in ventricular tissue. The Ca2+ influx raises the membrane potential to initiate the upstroke. Sodium entering would mainly depolarize in non-pacemaker heart tissue, potassium leaving would hyperpolarize/repolarize, and chloride influx isn’t responsible for this rapid depolarization.

The valve between the right ventricle and the pulmonary artery is the pulmonary valve. It is a semilunar valve located at the outflow tract from the right ventricle, and it opens to allow blood to flow into the pulmonary artery during systole. When the right ventricle relaxes (diastole), the valve closes to prevent blood from flowing back into the ventricle. This helps direct blood toward the lungs for oxygenation. In contrast, the aortic valve sits between the left ventricle and the aorta, the tricuspid valve lies between the right atrium and right ventricle, and the mitral valve lies between the left atrium and left ventricle.