A 5-week-old full-term boy is brought to the emergency department during winter due to intermittent respiratory pauses and cyanosis. He has had rhinorrhea and nasal congestion for the past 2 days. Over the past few weeks, the patient's father and older sibling have had nasal congestion and rhinorrhea but no fever or cough. The boy has received the hepatitis B vaccination and no other immunizations due to his young age. His temperature is 37.1 C (98.8 F). Physical examination shows bilateral crackles and wheezes and intermittent apnea. Leukocyte count is 9,000 cells/mm³, with 60% lymphocytes and 30% neutrophils. Which of the following is the most likely etiology of this patient's condition?

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Explanation: 11ec2b69_c1bf_1ac0_8f63_214e75fc63df_MD0005_00 This patient's clinical presentation in the setting of winter and household contacts with upper respiratory infection is concerning for respiratory syncytial virus (RSV) infection. RSV outbreaks cause uncomplicated nasal congestion and rhinorrhea in adults. Children age <2 years tend to have upper and lower respiratory tract involvement (eg, bronchiolitis). Infants age <2 months or those with a history of prematurity, congenital heart disease, and chronic lung disease are most susceptible to life-threatening apnea. (Choice A) Pertussis can be potentially lethal in infants due to relentless coughing fits, posttussive emesis, and apnea. Patients with pertussis typically have significant lymphocyte-predominant leukocytosis (>20,000/mm³ with >50% lymphocytes) and sick contacts with coughing paroxysms. (Choice B) Influenza can cause bronchiolitis and apnea in infants, but sick contacts with influenza typically have fever, body aches, and cough in addition to upper respiratory symptoms. (Choice C) Mycoplasma pneumoniae causes respiratory tract infection most often in school-aged children and young adults. The clinical presentation is usually gradual and is characterized by malaise, headache, fever, rhinorrhea, and sore throat with progression to an atypical pneumonia (eg, "walking pneumonia"). It does not normally cause bronchiolitis and is an uncommon pathogen in children age <5 years. (Choice D) Croup is an upper respiratory syndrome consisting of hoarseness, a barky cough, and inspiratory stridor due to upper airway inflammation. Parainfluenza virus is the most common causative organism. Educational objective: Respiratory syncytial virus is the most common cause of bronchiolitis. Outbreaks peak during the winter. Infants age <2 months are at high risk of apnea. References: Apnea in children hospitalized with bronchiolitis. (http://www.ncbi.nlm.nih.gov/pubmed/24101759) Identifying hospitalized infants who have bronchiolitis and are at high risk for apnea. (http://www.ncbi.nlm.nih.gov/pubmed/16997681) Clinical and laboratory features of pertussis in infants at the onset of a California epidemic. (http://www.ncbi.nlm.nih.gov/pubmed/21925678)

A 5-Week-Old Full-Term Boy Is Brought to the Emergency Department