Which of the following medications is indicated for preventing bradycardia in infants under one year old?

Atropine is indicated for preventing bradycardia in infants under one year old due to its effectiveness in blocking the effects of the vagus nerve on the heart. In young patients, particularly newborns and infants, bradycardia can occur in response to various stimuli, including increased vagal tone from airway manipulation, pain, or other stressors. Atropine works by inhibiting the action of acetylcholine on muscarinic receptors, which leads to an increase in heart rate. Given the physiological differences in the response of infants to bradycardia, atropine remains a crucial medication in emergency settings for this age group.

The other medications listed do not serve this specific purpose. Lidocaine is primarily an antiarrhythmic used in certain cardiac conditions rather than for bradycardia; succinylcholine and rocuronium are neuromuscular blocking agents, used to facilitate intubation or muscle relaxation, but they do not address heart rate issues directly. Hence, the selection of atropine specifically aligns with the need to manage bradycardia effectively in infants.