A 30-year-old woman on combined oral contraceptive pills (21 active pills + 7 placebo) experiences vaginal bleeding during the placebo week. What is the primary mechanism of this withdrawal bleeding?

• A Sudden drop in estrogen causing endometrial proliferation
• B Increased prostaglandin synthesis from rising estrogen levels
• C Rise in FSH triggering new follicular development
• D Progesterone withdrawal leading to spiral artery constriction and endometrial shedding ✓

Explanation

Withdrawal bleeding during the placebo week results from progesterone (and estrogen) withdrawal, causing spiral artery constriction, ischemia, and endometrial shedding. This mimics menstruation but is pharmacologically induced. Option A is wrong because estrogen drop would not cause shedding. This is a common exam question on contraceptive physiology.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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