<prompt> Which of the following is not a paranasal sinus ?Frontal sinusPyriform sinusEthmoid sinusMaxillary sinus  <response>: Ans. is b i.e., Pyriform sinus A sinus is a hollow air filled cavity. Paranasal sinuses are hollow air filled cavities in the skull which are connected to nasal cavity by a narrow hole in, the ostium. There are four pairs of these cavities and are often described as a unit which is refered to as paranasal sinuses. These four paranasal sinuses are :? i Frontal sinus in forehead iii Ethmoid sinus Between the eyes and nose ii Maxillary sinus Behind cheeks iv Sphenoid sinus Deep behind the ethmoid The four sinuses are divided into two groups Anterior group :- Include maxillary, frontal and anterior ethmoidal sinuses. All open in the midde meatus. Posterior group :- Include posterior ethmoidal which opens into superior meatus, and sphenoid sinus which opens into sphenoethmoidal recess. Pyriform sinus is a pa of hypopharynx laryngopharynx 