A molecule of neurotransmitter is to a(n) _____ as a key is to a keyhole.

• A. receptor site
• B. axon
• C. dendrite
• D. synapse

Answer: (A)

Verified step by step guidance

1. Understand the analogy presented in the problem: A molecule of neurotransmitter is being compared to a key, and we need to find what corresponds to a keyhole.
2. Recall the function of neurotransmitters in the nervous system. Neurotransmitters are chemical messengers that transmit signals across a chemical synapse, such as between a neuron and another cell.
3. Identify the role of a receptor site. A receptor site is a specific location on a neuron where neurotransmitters bind, similar to how a key fits into a keyhole.
4. Consider the options provided: receptor site, axon, dendrite, synapse. Determine which option functions as the 'keyhole' for the neurotransmitter 'key'.
5. Conclude that the receptor site is the correct answer, as it is the location where neurotransmitters bind, analogous to a key fitting into a keyhole.