Progesterone rises rapidly post-ovulation in the luteal phase

Steps:

• Day 1 of cycle is 5 February (menstruation start); ovulation occurs around day 14 (18 February).
• Luteal phase begins after ovulation (day 15 onward, ~19 February), when corpus luteum forms and secretes progesterone.
• Progesterone concentration increases most rapidly in early luteal phase (days 15-21, ~19-25 February).
• Option C (20-26 February) aligns with days 16-22, peak rise period.

Why C is correct:

• In the menstrual cycle, progesterone surges post-ovulation due to corpus luteum activity, per standard endocrinology (rises from low follicular levels to peak by mid-luteal phase).

Why the others are wrong:

• A: Menstrual/follicular phase (days 1-7); low progesterone, no rapid rise.
• B: Late follicular phase (days 8-14); estrogen dominates, progesterone minimal.
• D: Late luteal phase (days 23-28+); progesterone peaks then declines, not rapid increase.

Final answer: C