## Core Concept
Measles, also known as rubeola, is a highly contagious viral disease caused by the measles virus, which is a single-stranded RNA virus belonging to the *Paramyxoviridae* family. The disease is characterized by fever, cough, runny nose, and a distinctive rash. A key feature of measles is the presence of Koplik spots, which are small, white spots that appear inside the mouth.

## Why the Correct Answer is Right
The correct answer can be deduced by evaluating each statement for accuracy in relation to measles:
- The rash in measles typically starts on the face and then spreads to other parts of the body, including the trunk and limbs.
- Koplik spots are pathognomonic for measles and are seen on the buccal mucosa (inside the cheeks), not on the retina.
- Subacute sclerosing panencephalitis (SSPE) is a rare but serious long-term complication of measles.
- Measles is indeed caused by an RNA virus.
- The incubation period (I-P) of measles is approximately 10-14 days from exposure to the onset of symptoms, but the prodromal phase (which includes symptoms like fever, cough, and Koplik spots) usually lasts about 2-3 days before the rash appears.

## Why Each Wrong Option is Incorrect
- **Option A:** The statement that "Rash appears first on leg" is incorrect because the rash in measles typically appears first on the face and then spreads to the rest of the body, including the legs.
- **Option B:** The statement that "Koplik spot are seen on retina" is incorrect because Koplik spots are actually seen on the buccal mucosa, not on the retina.
- **Option D:** and **Option E:** are accurate statements regarding measles.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that Koplik spots are a diagnostic feature of measles and appear as small, white spots on the buccal mucosa. They are often described as "blue-white" spots and are seen 1-2 days before the onset of the rash. Recognizing these spots can lead to early diagnosis and management of measles.

## Correct Answer: B. Koplik spot are seen on retina