<prompt> Please answer with one of the option in the bracket. Write reasoning in between . Write answer in between .Q:A previously healthy 6-week-old infant is brought to the emergency department because of fever, fatigue, and dry cough for the past 24 hours. She has been feeding poorly and has had difficulty latching on when breastfeeding since this started. She has also had nasal congestion. The mother reports that her daughter has not been going through as many diapers as usual. She was born by uncomplicated vaginal delivery at 42 weeks gestation. Her mother is a cystic fibrosis carrier. The patient has been treated with acetaminophen for the last 24 hours and vitamin D drops since birth. She appears irritable, pale, and lethargic. She is at the 25th percentile for both length and weight; she had the same percentiles at birth. Her temperature is 38.2C 100.7F, and the respirations are 64min. Pulse oximetry on room air shows an oxygen saturation of 92. Examination shows an ill-appearing infant with a cough and nasal flaring. Mucous membranes are dry. Chest examination shows intercostal and supraclavicular retractions. Expiratory wheezes are heard on auscultation. Which of the following is the most likely causal organism?? A: Parainfluenza virus, B: Respiratory syncytial virus, C: Rhinovirus, D: Streptococcus agalactiae, E: Streptococcus pneumoniae,  <response>: This is a question about identifying the most likely causal organism in a 6-week-old infant presenting with fever, fatigue, dry cough, poor feeding, difficulty latching, nasal congestion, decreased urine output, irritability, lethargy, low oxygen saturation, cough, nasal flaring, dry mucous membranes, intercostal and supraclavicular retractions, and expiratory wheezes. Key factors:- 6-week-old infant - Fever, cough, congestion, poor feeding - suggests respiratory infection- Decreased urine output - suggests dehydration- Low oxygen saturation - suggests lower respiratory involvement- Intercostal retractions - suggests lower respiratory disease- Expiratory wheezes - suggests bronchiolitis Based on the clinical presentation, the most likely diagnosis is bronchiolitis, most commonly caused by respiratory syncytial virus RSV in this age group. Other less likely options are parainfluenza virus, rhinovirus, Group B strep, and pneumococcus. However, the typical presentation of RSV bronchiolitis makes it the most likely organism.B: Respiratory syncytial virus 