Okay, let's tackle this epidemiology question. The user is asking about calculating the incidence of myopia in a town. First, I need to recall the difference between prevalence and incidence. Prevalence is the total number of existing cases in a population at a specific time, while incidence is the number of new cases that develop over a certain period.

The question gives a population of 5000. On January 1, 2011, there are 500 existing myopic individuals. By December 31, 2011, 90 new cases are added. The question is asking for the incidence rate for 2011.

To calculate incidence, I remember that it's the number of new cases divided by the population at risk during that period. The population at risk would be the total population minus those already affected. So that's 5000 - 500 = 4500. Then, the incidence rate is 90 new cases divided by 4500, which gives 0.02, or 2%.

Now, looking at the options (even though they aren't listed here), the correct answer should be 2%. The other options might be miscalculations. For example, someone might mistakenly divide by the total population (5000) instead of the at-risk population (4500), leading to 1.8%. Others might confuse prevalence with incidence, adding existing and new cases (500+90=590) over 5000, which is 11.8%. Alternatively, using the total population including existing cases (5000) for incidence would also be wrong.

The key point here is understanding that incidence focuses only on new cases among those who were not previously affected. This is a common exam trap where students mix up prevalence and incidence. So the clinical pearl is to always subtract existing cases from the total population when calculating incidence to get the correct at-risk population.

**Core Concept**
This question tests understanding of **epidemiological incidence** versus prevalence. **Incidence** measures new cases of a disease in a population at risk over a specific time period, calculated as:
*Incidence = (Number of new cases during period) / (Population at risk during period)*.

**Why the Correct Answer is Right**
The population at risk is 5000 total residents minus 500 existing myopic individuals (4500). There are 90 new cases in 2011. Thus:
*Incidence = 90 / 4500 = 0.02 (2%)*.
This reflects the rate at which new myopia cases develop in non-myopic individuals.

**Why Each Wrong Option is Incorrect**
**Option A:** Likely miscalculates incidence by using total population (5000), giving 1.8% (90/5000).
**Option B:** May add existing (500) and new cases (90) to calculate prevalence (590/5000 = 11.8%), not incidence.
**Option D:** Could incorrectly report 90/5000 (1.8%) if prevalence is confused with incidence

✓ Correct Answer: B. 0.02