When stable SVT persists after vagal maneuvers, the drug of choice is adenosine because it acutely and briefly blocks AV nodal conduction, which interrupts the reentrant circuit that most commonly involves the AV node. Adenosine works so quickly and has a very short half-life, so its effects wear off within seconds, allowing a rapid return to sinus rhythm if the SVT is AV nodal dependent. The typical approach is a rapid IV push of 6 mg, followed by a flush; if the rhythm does not convert, a second dose of 12 mg can be given. Mechanistically, adenosine activates A1 receptors in the AV node, opening potassium channels and hyperpolarizing the nodal cells. This transiently halts conduction through the AV node, effectively terminating the reentrant loop that maintains the tachycardia. Other options are less ideal in this immediate context. Atropine is used for symptomatic bradycardia, not for terminating SVT. Metoprolol can slow AV nodal conduction, but it is slower to act and may not terminate the tachycardia as reliably or rapidly as adenosine. Amiodarone has broader antiarrhythmic uses and is more appropriate for refractory tachyarrhythmias or wide-complex tachycardias, rather than the initial pharmacologic step in classic AV nodal–dependent SVT.

Reducing inflammation quickly to protect the coronary arteries is the goal in Kawasaki disease. The best initial therapy is giving intravenous immunoglobulin in a single high-dose infusion together with high-dose aspirin. IVIG dampens the immune attack on the vessel walls, substantially lowering the risk of coronary artery aneurysms. High-dose aspirin helps control inflammation and fever and provides antiplatelet effect to prevent thrombosis if aneurysms develop. After fever subsides, aspirin is typically continued at a low, antiplatelet dose for a period determined by coronary findings. Antibiotics alone won’t address the vasculitis, corticosteroids alone aren’t first-line for aneurysm prevention, and doing nothing would allow ongoing vascular inflammation SAMPLEand higher aneurysm risk.

Red currant jelly stools signal intussusception. When a segment of bowel telescopes into an adjacent segment, the outflow venous return is obstructed, causing venous congestion, edema, and mucosal ischemia. Blood and mucus mix in the lumen, producing the characteristic reddish, jelly-like stool. This finding is classic in young children with intermittent abdominal pain, vomiting, and sometimes a palpable abdominal mass. In contrast, malrotation with volvulus usually presents with bilious vomiting and signs of acute distress due to obstruction; Meckel diverticulum can cause painless lower GI bleeding from ectopic gastric mucosa; pyloric stenosis leads to projectile nonbilious vomiting and an olive-sized mass in the epigastrium.

Pneumoperitoneum with peritoneal signs means a hollow viscus perforation causing peritonitis, a surgical emergency. The priority is source control, so an emergent laparotomy to locate and repair the perforation (with lavage and drainage as needed) is the appropriate management. Conservative approaches are unsuitable here because peritonitis from perforation carries high risk of sepsis. Broad-spectrum IV antibiotics and aggressive fluid resuscitation should be started, and the patient is kept NPO while preparing for surgery. While laparoscopy can be considered in select stable patients, the classic teaching for a patient with diffuse abdominal pain, peritoneal signs, and free intraperitoneal air is urgent open exploratory surgery to repair the perforation.