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

Peussis is an acute highly contagious respiratory tract infection caused by Bordetella peussis. Extremely contagious and clinical presentation can be divided into 3 stages like catarrhal, paroxysmal and convalescent It is called whooping cough due to the characteristic whoop produced by the air rushing in during inspiration through half open glottis. - diagnosis is mainly based on clinical symptoms. - the sample of choice is nasopharyngeal swab. Reference : Ghai essential pediatrics, 9 th edition, pg no:237