**Core Concept**
Measles is a highly contagious viral infection that poses significant public health risks, particularly in developing countries with limited access to vaccination and healthcare resources. The mortality rate of measles is influenced by various factors, including the effectiveness of the immune response, the severity of secondary infections, and the availability of medical care.

**Why the Correct Answer is Right**
The mortality rate of measles in developing countries is estimated to be around 10%. This is due to several factors, including the high frequency of secondary bacterial infections, such as pneumonia and diarrhea, which can lead to severe complications and death. In addition, the measles virus can cause encephalitis, a potentially life-threatening condition that can occur in approximately 1 in 1,000 cases. The mortality rate is also influenced by the age of the individual, with infants under 1 year of age being the most vulnerable.

**Why Each Wrong Option is Incorrect**
**Option B:** 20% - This is an overestimation of the mortality rate in developing countries, and it is not supported by evidence from reputable sources.
**Option C:** 30% - This option is also incorrect, as it is significantly higher than the estimated mortality rate in developing countries.
**Option D:** 40% - This option is an extreme overestimation and is not supported by any credible sources.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the measles mortality rate can be significantly reduced through vaccination, particularly in developing countries. According to the World Health Organization (WHO), widespread measles vaccination has led to a 90% reduction in measles deaths worldwide since 2000.

**✓ Correct Answer: A. 10%**