**Core Concept**
Combined Oral Contraceptive Pills (OCPs) contain a combination of estrogen and progestin to prevent ovulation. The minimum dose of estrogen required for effective contraception is a pharmacological principle that balances efficacy and safety.

**Why the Correct Answer is Right**
The minimum dose of estrogen in combined OCPs is typically 20 micrograms of ethinyl estradiol (EE) or its equivalent. This low-dose estrogen formulation has been shown to be as effective as higher-dose formulations in preventing ovulation and pregnancy. The lower estrogen dose reduces the risk of adverse effects, such as thromboembolism and endometrial cancer. The estrogen in OCPs works by inhibiting the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn reduces the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the pituitary gland.

**Why Each Wrong Option is Incorrect**
**Option A:** Higher doses of estrogen (e.g., 50 micrograms of EE) were used in older formulations but are no longer necessary for effective contraception and increase the risk of adverse effects.
**Option B:** Progestin-only pills do not contain estrogen and are not considered combined OCPs.
**Option C:** The minimum dose of estrogen in combined OCPs is not 10 micrograms of EE, as this dose may not be effective in preventing ovulation and pregnancy.

**Clinical Pearl / High-Yield Fact**
Remember that the minimum dose of estrogen in combined OCPs is 20 micrograms of EE, and that lower-dose estrogen formulations are generally safer and just as effective as higher-dose formulations.

**Correct Answer:** C. 10 micrograms of EE