## Core Concept
The question assesses understanding of the impact of sequential testing on the sensitivity and specificity of a screening method for colonic cancer. The Hemoccult test is used as a primary screening tool, followed by the Hemoccult II test for those who test positive. This approach involves understanding how two-step testing affects the overall sensitivity and specificity of the screening process.

## Why the Correct Answer is Right
The correct answer, , indicates that the sequential testing strategy decreases the sensitivity but increases the specificity.
- **Sensitivity** refers to the test's ability to correctly identify those with the disease (true positive rate).
- **Specificity** refers to the test's ability to correctly identify those without the disease (true negative rate).
In a two-step testing strategy like this, where a positive result on the first test is followed by a second test:
- A decrease in **sensitivity** occurs because some individuals with the disease might test negative on the second test, even if they tested positive on the first.
- An increase in **specificity** occurs because those without the disease are less likely to test positive on both tests, reducing false positives.

## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests an increase in sensitivity and specificity, which is not accurate for a sequential testing strategy that requires two positive tests to refer for evaluation. The sensitivity decreases because some true positives may be missed in the second test.
- **Option B:** This option suggests an increase in sensitivity and a decrease in specificity, which is the opposite of what occurs. The specificity increases because the strategy becomes more selective.
- **Option D:** This option suggests a decrease in both sensitivity and specificity, which is incorrect because the specificity increases with sequential testing.

## Clinical Pearl / High-Yield Fact
A key point to remember is that sequential testing, or using a second, more specific test after a positive initial screen, **increases the positive predictive value (PPV) of the testing strategy**. This means that a positive result from the two-step process is more likely to be a true positive. However, it's crucial to balance this with the potential decrease in sensitivity, ensuring that not too many cases are missed.

## Correct Answer: C.