Final answer:
An acetylcholinesterase inhibitor can cause prolonged signaling in a muscle cell at the neuromuscular junction.
Step-by-step explanation:
Prolonged signaling in a muscle cell at the neuromuscular junction can be caused by an acetylcholinesterase inhibitor, which prevents the breakdown of acetylcholine. When acetylcholine is not broken down, it continues to bind to acetylcholine receptors and transmit signals, leading to prolonged signaling.