Step 1: Understanding the Concept:
A "pathognomonic sign" is a symptom that is specifically characteristic of a particular disease and can be used to make a definitive diagnosis.
Step 2: Detailed Explanation:
Koplik's spots are small, white spots (often compared to grains of salt) on a reddened background that appear on the inside of the cheeks.
They typically appear 2 to 3 days before the measles rash begins.
Their presence is diagnostic for Measles (Rubeola).
Rubella (German measles) shows Forchheimer spots (petechiae on the soft palate), not Koplik's spots.
Step 3: Final Answer:
Koplik's spots are the hallmark sign of Measles.