The most likely cause of bronchiolitis in a 6 months old infant is:

Option 3 RESPIRATORY SYNCYTIAL VIRUS Infants: RSV is the most common cause of lower respiratory tract infection below 1 year of age, causing bronchiolitis, pneumonia, and tracheobronchitis. Bronchiolitis is usually self-limiting, but may cause a serious disease in premature infants, in immunocompromised hosts, and in individuals with underlying lung diseases. Option 1, 2 Measles virus and Mumps virus do not cause bronchiolitis. Option 4 Human parainfluenza viruses are one of the major causes of lower respiratory tract disease: Common cold syndrome such as rhinitis and pharyngitis is the MC presentation Croup (laryngotracheobronchitis): Seen with type 1 and 2 and involves older children Pneumonia or bronchiolitis: Seen with type 3, but less common than RSV.