Here is the passage again:
Dr. Gregory surveyed the auditorium full of nervous, sweating biochemistry
students. Her gaze often stopped on Bryan. Whose
test anxiety manifested in the loud popping of his knuckles and the high-speed
bouncing of his left leg.
The third part is a subordinate clause fragment. Although it has a subject, anxiety, and a verb, manifested, the relative pronoun whose keeps it from making a complete thought.