Key Concept: Estrus synchronization aligns a group of females into heat together so they can all be bred/AI'd on a fixed schedule.
Why it's correct: Prostaglandin F2α is injected to regress the corpus luteum (CL); once the CL breaks down, progesterone falls sharply and the animal returns to estrus within about 2–5 days — a fast, predictable trigger.
Why others fail: Prolactin, insulin, and thyroxine have no role in regressing the CL or triggering heat — they affect lactation, metabolism, and growth respectively, not the reproductive cycle.