**Core Concept**
Post-tetanic potentiation is a phenomenon observed in the nervous system where the amplitude of the action potential increases after a series of high-frequency stimuli, such as a tetanic stimulation. This occurs due to the depletion of presynaptic neurotransmitter vesicles and the subsequent increase in the release of neurotransmitters. The process involves changes in the presynaptic terminal, particularly the calcium channels and the release machinery.

**Why the Correct Answer is Right**
Post-tetanic potentiation is primarily due to the increased amount of calcium ions (Ca²⁺) entering the presynaptic terminal, which triggers the release of more neurotransmitters. As the presynaptic terminal is depleted of vesicles, the nerve terminal undergoes a process called "calcium-dependent facilitation," where the calcium channels become more sensitive to the influx of calcium ions. This results in an increase in the release of neurotransmitters, leading to an enhanced postsynaptic potential.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might suggest that post-tetanic potentiation is due to a change in the postsynaptic receptor density or affinity, which is not accurate. While changes in receptor density or affinity can affect synaptic transmission, they are not the primary cause of post-tetanic potentiation.
**Option B:** This option might imply that post-tetanic potentiation is due to an increase in the number of action potentials, which is not the case. Post-tetanic potentiation is a result of the increased release of neurotransmitters, not an increase in the frequency of action potentials.

**Clinical Pearl / High-Yield Fact**
Post-tetanic potentiation is an important concept in understanding the physiology of the nervous system, particularly in the context of neuromuscular transmission. It has implications for the treatment of neuromuscular disorders, such as myasthenia gravis, where the use of anticholinesterase inhibitors can enhance neuromuscular transmission.

**Correct Answer: C. Increased calcium-dependent facilitation**