## Mechanism of Action: The Key Discriminator **Key Point:** Mifepristone and misoprostol differ fundamentally in their pharmacological mechanism, which is the most reliable discriminating feature between these two cervical ripening agents. ### Mifepristone (RU-486) - Competitive antagonist of progesterone receptors - Blocks progesterone-mediated maintenance of pregnancy - Sensitizes myometrium to prostaglandins - Requires 24–48 hours for cervical ripening effect - Used primarily in medical abortion; limited role in induction of labor in India ### Misoprostol (Cytotec) - Synthetic prostaglandin E₁ analogue - Directly stimulates myometrial contractions and cervical ripening - Rapid onset: contractions within 30 minutes to 1 hour - Widely available, inexpensive, stable at room temperature - Standard agent for cervical ripening in most Indian centers ## Comparative Table | Feature | Mifepristone | Misoprostol | | --- | --- | --- | | **Mechanism** | Progesterone antagonist | Prostaglandin E₁ analogue | | **Onset of action** | 24–48 hours | 30 min–1 hour | | **Route** | Oral | Oral, vaginal, sublingual | | **Hyperstimulation risk** | Lower | Moderate to high | | **Cost** | Higher | Lower | | **Availability in India** | Limited | Widely available | | **Prior LSCS** | Relative contraindication | Absolute contraindication | **High-Yield:** The mechanism of action (progesterone antagonism vs. prostaglandin agonism) is the single best discriminating feature tested in NEET PG. Both agents ripen the cervix, but through entirely different pathways. **Clinical Pearl:** Mifepristone is rarely used for labor induction in India due to limited availability, higher cost, and slower onset. Misoprostol remains the gold standard for cervical ripening when oxytocin alone is insufficient.
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