**Core Concept**
The minimum effective dose of Ethinyl estradiol in combination oral contraceptive pills (COCs) refers to the lowest amount of estrogen required to achieve effective contraception while minimizing the risk of side effects. Ethinyl estradiol is a synthetic form of estrogen that plays a crucial role in the regulation of ovulation and the maintenance of pregnancy.

**Why the Correct Answer is Right**
The minimum effective dose of Ethinyl estradiol in combination oral pills is 20 mcg (micrograms), which is often used in low-dose contraceptives. This dose is sufficient to prevent ovulation and thicken cervical mucus, thereby preventing sperm from reaching the egg. The use of 20 mcg of Ethinyl estradiol has been shown to be as effective as higher doses in preventing pregnancy while reducing the risk of side effects such as nausea, breast tenderness, and blood clots.

**Why Each Wrong Option is Incorrect**
**Option B:** 35 mcg is a common dose of Ethinyl estradiol used in many combination oral contraceptives, but it is not the minimum effective dose.
**Option C:** 50 mcg and **Option D:** 75 mcg are higher doses of Ethinyl estradiol, which are no longer considered effective as minimum doses due to the availability of lower-dose options.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the minimum effective dose of Ethinyl estradiol can vary among individuals, and some women may require higher doses due to factors such as age, weight, or medical history. However, starting with the lowest effective dose can help minimize side effects and improve patient compliance.

**✓ Correct Answer: A. 20 mcg**