You are answering a USMLE-style medical question. Question: An investigator is studying the pattern of glutamate release from presynaptic nerve terminals in human volunteers with Alzheimer disease. The concentration of glutamate in the CA1 region of the hippocampus is measured using magnetic resonance spectroscopy after Schaffer collateral fibers are electrically stimulated. Which of the following events most likely occurs immediately prior to the release of neurotransmitters? Options: A) Activation of G protein-coupled receptors B) Accumulation of cAMP C) Opening of ligand-gated ion channels D) Influx of calcium Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.