A child presents with recurrent bouts of severe cough followed by an audible whoop. Which of the following is considered the best type of specimen to isolate the organism and confirm the diagnosis?

Child in the question is showing features of whooping cough. Definitive diagnosis of whooping cough can be made after isolating Bordetella peussis from the respiratory tract. Either nasopharyngeal swab or nasopharyngeal aspirate is taken for sampling. Ref: Whooping Cough By Patrick G. Guilfoile, David Heymann, Hilary Babcock page 56-65; Communicable disease control handbook By Jeremy Hawker page 238.