Ulipristal acetate (UPA) used as emergency contraception differs from levonorgestrel in its mechanism because:

• A UPA is an estrogen receptor antagonist while levonorgestrel is a progestogen
• B UPA acts by preventing fertilization through sperm toxicity, while levonorgestrel delays ovulation
• C UPA is only effective within 24 hours of intercourse while levonorgestrel is effective up to 72 hours
• D UPA is a selective progesterone receptor modulator (SPRM) that inhibits ovulation even after the LH surge begins, while levonorgestrel is only effective before the LH surge ✓

Explanation

Ulipristal acetate is a selective progesterone receptor modulator that inhibits or delays follicle rupture even after the LH surge has commenced by blocking progesterone's stimulatory effect on LH surge amplification; levonorgestrel primarily works by inhibiting/delaying ovulation before or at the time of the LH surge but is ineffective once the surge has begun. This mechanistic difference gives UPA superior efficacy up to 120 hours post-coitally and in the periovulatory period. UPA is effective up to 5 days (120 hours).

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.