**Question:** Emergency contraception prevents pregnancy by all of the following mechanisms, except:

A. Sterilization
B. Directly affecting fertilization
C. Altering the endometrium to make it unfit for implantation
D. Inhibiting ovulation

**Core Concept:** Emergency contraception (EC) is a method of post-coital contraception that aims to prevent or delay pregnancy when used after unprotected sex or when contraceptive methods have failed. EC works through various mechanisms to minimize the chances of fertilization, implantation, and/or pregnancy progression.

**Why the Correct Answer is Right:** Sterilization (Option A) is not a suitable mechanism for emergency contraception because it permanently prevents conception, which is not its purpose. Emergency contraception is meant to be used only once or a few times, not as a permanent contraceptive method.

**Why Each Wrong Option is Incorrect:**

B. Directly affecting fertilization (Option B) is not the primary mechanism of action for EC pills or copper IUDs. While these methods may indirectly influence fertilization, their primary mode of action lies in preventing implantation and/or promoting uterine contractions to expel the fertilized egg.

C. Altering the endometrium to make it unfit for implantation (Option C) is an incorrect mechanism for EC. Emergency contraception primarily focuses on preventing fertilization and/or promoting uterine contractions, rather than changing the endometrium.

D. Inhibiting ovulation (Option D) is a correct mechanism for EC pills, such as levonorgestrel pills, but it is not a mechanism for copper IUDs. Copper IUDs primarily work by creating a hostile environment for sperm and the fertilized egg within the uterus.

**Clinical Pearl:** Emergency contraception is a valuable tool for preventing unplanned pregnancies and reducing the risk of sexually transmitted infections. It is essential for healthcare providers to educate patients about the available options (pills and IUDs) and their mechanisms of action to ensure appropriate selection and use. Timely administration of EC is crucial for its effectiveness.