**Core Concept**
The Breteau index is a measure used in the assessment and control of urban malaria transmission. It is a tool used to estimate the intensity of malaria transmission in urban areas, taking into account the number of breeding sites and the number of mosquitoes collected in these sites.

**Why the Correct Answer is Right**
The Breteau index is calculated by dividing the number of positive breeding sites (i.e., sites where Anopheles larvae are found) by the total number of inspected breeding sites, multiplied by 100. This index is a useful metric for identifying areas with high malaria transmission potential and for monitoring the effectiveness of control measures. The Breteau index takes into account the presence of Anopheles larvae in breeding sites, such as ponds, lakes, and other bodies of water, which are potential sources of malaria transmission in urban areas.

**Why Each Wrong Option is Incorrect**
**Option A:** The Breteau index is not used for calculating the incidence of malaria cases.
**Option B:** The Breteau index is not a measure of the prevalence of malaria in a population.
**Option C:** The Breteau index is not a measure of the effectiveness of malaria control measures.

**Clinical Pearl / High-Yield Fact**
The Breteau index is a useful tool for identifying areas with high malaria transmission potential, but it should be used in conjunction with other metrics, such as the Anopheles larval density and the parasite rate, to get a comprehensive picture of malaria transmission in urban areas.

**Correct Answer:** A. The Breteau index is used to estimate the intensity of malaria transmission in urban areas.