If acetylcholinesterase became mutated and nonfunctional, which of the following would most likely occur in the synaptic cleft?

Step 3: Consider the consequences of impaired acetylcholine breakdown. Without degradation, acetylcholine will accumulate in the synaptic cleft, leading to prolonged activation of acetylcholine receptors on the postsynaptic neuron.

Step 4: Evaluate the incorrect options. Decreased levels of acetylcholine due to increased degradation cannot occur because the enzyme responsible for degradation is nonfunctional. Enhanced reuptake into the presynaptic neuron is unrelated to acetylcholinesterase activity. Increased synthesis of acetylcholinesterase is not a direct consequence of the mutation described.