Ans. is 'd' i.e., Parainfluenza virusCroup (Larvngotracheobronchitis)o Laryngotracheobronchitis is the most common infectious cause of obstruction in children usually occurring between the ages of 6 months and 3 years.o Male children (boys) are characteristically more frequently involved than females (girls)Edologyo It is a viral infection most frequently caused by Parainfluenza virusPathologyo The most characteristic pathological feature is edema formation in the subglottic areao The loose areolar tissue in the subglottic area swells up and causes predominant signs of upper airway obstruction.Presentationo Gradual onset with a prodrome of upper respiratory? Symptomso Hoarseness and barking cough (croupy cough)o Stridor (initially inspiratory than biphasic)o Fever is usually low grade (or absent) although may occasionally be high gradeo Droolings is characteristically absent and there is no dysphagia (seen in epiglottitis)Imaging (X ray)o Symmetric 'steeple' or 'funnel shaped' narrowing of the subglottic region (steeple sign)o Hypopharyngeal widening or distensiono Normal epiglottis and arvepiglottic foldsTreatmento Mild symptoms (barking cough but no stridor at rest)Supportive therapy alone with humified oxygen, oral hydration and minimal handling,o Moderate symptoms (barking cough with stridor at rest)Active intervention with humified oxygen, Nebulized racemic epinephrine and glucocorticoids (steroids),o Severe symptoms (impending respiratory failure)Require an artificial airway (Intubation with endotracheal tube or tracheostomy may be required).Antibiotics are not routinely indicated in the treatment of Acute Laryngotracheobronchitis.Their use is limited if there is evidence of secondary bacterial infection