**Core Concept**
The sensitivity of a diagnostic test is the proportion of true positives it correctly detects among all actual positives in the population. In this context, the sensitivity of the test for detecting trisomy 21 (Down's syndrome) is being evaluated.

**Why the Correct Answer is Right**
To calculate the sensitivity, we need to identify the true positives (women with trisomy 21 who test positive) and the actual positives (women with trisomy 21). From the given data, there are 100 women with trisomy 21 and 50 of them tested positive. Therefore, the sensitivity of the test is the number of true positives (50) divided by the total number of actual positives (100 + 50 = 150) is incorrect. The correct calculation is 50/100.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a valid calculation for sensitivity. Sensitivity is the proportion of true positives among all actual positives, not a ratio of true positives to the total number of women tested.

**Option B:** This option is incorrect because it does not accurately represent the calculation for sensitivity. Sensitivity requires the number of true positives and the total number of actual positives.

**Option C:** This option is incorrect because it incorrectly calculates the sensitivity as the proportion of true positives among all women tested (including both actual positives and negatives).

**Option D:** This option is incorrect because it is a calculation error. It incorrectly divides the number of true positives by the total number of women with trisomy 21 rather than the total number of actual positives.

**Clinical Pearl / High-Yield Fact**
When evaluating the sensitivity of a diagnostic test, it's essential to understand the relationship between the test results and the actual disease status. In this case, the sensitivity of the test is a measure of its ability to detect trisomy 21 correctly.

**Correct Answer:** A. 50/100 is incorrect. The correct sensitivity is 50%.