<prompt> A child with three days history of upper respiratory tract infection presents with stridor, which decreases on lying down position. What is the most probable diagnosisAcute EpiglottitisLaryngotracheobronchitisForeign body aspirationRetropharyngeal abscess  <response>: This child has : -3 days history of upper respiratory tract infection.Followed by stridorThese features suggest the diagnosis of croup.Clinical manifestations of Croup Most patients have an upper respiratory tract infection with some combination of -Rhinorrhea A mild cough Pharyngitis Low-grade fever After 1-3 days signs and symptoms of upper respiratory tract obstruction become apparent -Barking coughHoarseness Inspiratory stridor Symptoms are characteristically worse at night.Symptoms often recur with decreasing intensity for several days and resolve completely in one week.Agitation and crying aggravate the symptoms.The child may prefer to sit up in bed or be held upright.Chest X-rayCharacteristic narrowing of the subglottic region known as the steeple sign. 