Questions: What ion enters a neuron causing depolarization of the cell membrane? Select one: a. PHOSPHATE b. POTASSIUM c. CHLORIDE d. SODIUM

The answer is the last one (or D): SODIUM.

Explanation for each option: a. PHOSPHATE: Phosphate ions do not play a primary role in the depolarization of the neuron cell membrane. They are more involved in energy transfer and storage within the cell. b. POTASSIUM: Potassium ions (K+) are involved in repolarization and maintaining the resting membrane potential, but they do not cause depolarization. During depolarization, potassium ions actually move out of the cell. c. CHLORIDE: Chloride ions (Cl-) are generally involved in hyperpolarization, making the inside of the cell more negative, which is the opposite of depolarization. d. SODIUM: Sodium ions (Na+) enter the neuron during depolarization. When a neuron is stimulated, sodium channels open, allowing Na+ to flow into the cell, making the inside of the cell more positive and leading to depolarization.

Summary: Depolarization of the neuron cell membrane is primarily caused by the influx of sodium ions (Na+). Therefore, the correct answer is D: SODIUM.