**Core Concept**
The Pearl Index is a statistical measure used to express the effectiveness of a contraceptive method. It is defined as the number of failures (pregnancies) per 100 woman-years of exposure.

**Why the Correct Answer is Right**
The Pearl Index is calculated by dividing the number of unintended pregnancies by the total number of woman-years of exposure, and then multiplying by 100. This gives a rate of failures per 100 woman-years, which is a standardized measure of contraceptive efficacy. The Pearl Index is widely used in clinical trials and epidemiological studies to compare the effectiveness of different contraceptive methods.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the Pearl Index is not used to calculate the efficacy of other medical treatments, such as antibiotics or antihypertensives.
**Option B:** This option is incorrect because the Pearl Index is not a measure of contraceptive effectiveness in terms of prevention of sexually transmitted infections (STIs).
**Option C:** This option is incorrect because the Pearl Index is not a measure of the side effects or adverse effects of contraceptive methods.

**Clinical Pearl / High-Yield Fact**
The Pearl Index is an important measure of contraceptive efficacy, and it is essential to understand how it is calculated in order to critically evaluate the effectiveness of different contraceptive methods.

**Correct Answer: A**. The Pearl Index is used to calculate the effectiveness of contraceptive methods.