A 26-year-old woman with regular 28-day menstrual cycles undergoes endometrial biopsy on day 13 and day 24 of her cycle. The day 13 biopsy shows straight tubular glands with mitotic figures amid proliferative stroma; the day 24 biopsy reveals tortuous glands with abundant secretions and edematous stroma. If no fertilization of the oocyte occurs, which of the following hormonal events most directly initiates the breakdown of the endometrium and the onset of menses?