Koplik spots are small, white or bluish-white lesions on an erythematous base found on the buccal mucosa opposite the molars. They are considered pathognomonic for measles and typically appear 1-2 days before the characteristic skin rash. Their presence helps in the early clinical diagnosis of measles, distinguishing it from other viral exanthems. These spots are a hallmark early sign and are not seen in mumps, rubella, or chickenpox.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson et al., 20th Edition.