Which pharmaceutical agent should not be used during a bronchoscopy when a child is coughing?

In the context of a bronchoscopy, succinylcholine is a neuromuscular blocking agent that can cause paralysis of the muscles, including those involved in breathing. Its use during a procedure where the child is actively coughing would be contraindicated because the goal is to maintain airway protection and respiration. Coughing is a protective reflex that clears secretions from the airway; administering a drug that paralyzes these muscles can lead to respiratory distress or aspiration if the airway is compromised.

In contrast, the other options serve different roles. Cetacaine is a topical anesthetic that can help minimize discomfort during the procedure. Racemic epinephrine is often used for its vasoconstrictive properties to reduce airway edema and is safe in this context. Ketamine, an anesthetic, has properties that provide analgesia and can allow for sedation while maintaining airway reflexes. Hence, succinylcholine's muscle-paralyzing effect is precisely what makes it inappropriate for use during a bronchoscopy in an actively coughing child.