## **Core Concept**
Measles, also known as rubeola, is a highly contagious viral infection caused by the measles virus, which belongs to the *Paramyxoviridae* family. It is characterized by fever, cough, runny nose, and a distinctive rash. The measles virus is an RNA virus.

## **Why the Correct Answer is Right**
The correct answer involves verifying each statement:
- **c) Long term complication follows in form of SSPE**: Subacute sclerosing panencephalitis (SSPE) is a rare but serious long-term complication of measles infection, occurring years after the initial infection. It results from a persistent measles virus infection in the brain.
- **d) Caused by RNA virus**: The measles virus is indeed an RNA virus, belonging to the *Paramyxoviridae* family.
- **e) IP–IS 2–3 day**: The incubation period (IP) of measles, which is the time from exposure to the onset of symptoms, is approximately 10-14 days, but the prodromal phase (which includes symptoms like fever, cough, and Koplik spots) typically lasts about 2-3 days before the rash appears.

## **Why Each Wrong Option is Incorrect**
- **Option A: a) Rash appear first on leg**: The rash in measles typically starts on the face and then spreads to other parts of the body, including the trunk and limbs. It does not characteristically appear first on the legs.
- **Option B: b) Koplik spots are seen in retina**: Koplik spots are small, white spots that appear on the buccal mucosa (the inside of the cheeks) during the prodromal phase of measles, not in the retina.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Koplik spots** are pathognomonic for measles and appear as small, white spots on the buccal mucosa. They are an important diagnostic clue, appearing before the rash and resolving once the rash appears.

## **Correct Answer: C.**