The near-instantaneous depolarization of the oocyte membrane that prevents additional sperm from fusing, lasting about a minute, is called the:

1. The oocyte deploys two sequential mechanisms to prevent polyspermy after the first sperm fuses. 2. The first mechanism acts almost instantly, changing sodium permeability and depolarizing the membrane. 3. This near-instant, roughly one-minute-long mechanism is called the fast block. 4. Slow block and Cortical reaction alone are both different figures a reader might guess from memory, but neither matches the specific figure given here. 5. Zona hardening is also incorrect for the same reason: it does not match the specific figure given here. 6. The slower, longer-lasting cortical reaction (slow block) follows afterward, so it is ruled out as the near-instantaneous mechanism. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1321 §The Zygote_