Final answer:
The indication of pertussis in an adolescent is a dry, hacking cough that is severe and sometimes followed by a 'whoop' sound after coughing.
Step-by-step explanation:
The finding that indicates pertussis, also known as whooping cough, in an adolescent with an upper respiratory tract infection is a dry, hacking cough. This severe coughing is often followed by a distinctive 'whoop' sound as air is drawn through the inflamed airway. Pertussis is a result of an infection with the bacterium Bordetella pertussis and can be particularly severe in infants and children. It is highly communicable, especially during the catarrhal stage, which is characterized by inflammation of the mucous membranes and excessive secretions. The other options listed, such as inflamed throat with exudate, purulent eye drainage, and Koplik spots on buccal mucosa, are indicative of other infections but not specifically pertussis.