Q. A 6-month-old female presents to the emergency department with respiratory distress. Her parents report the patient developed a cough 2 days ago. She has reportedly tolerated feedings and maintained 2 to 3 wet diapers per day. Her mother states the patient’s temperature was 101 degrees last night. Earlier today, the patient’s cry reportedly became suddenly hoarse, and she “sounded like a seal barking” when she coughed. The parents are extremely anxious and ask if their baby is “dying”. They report she was the product of a term-delivery, her immunizations are up to date, and she has no known medical problems. On physical examination, the patient appears dyspneic and agitated. Her temperature is 37.2 degrees Celsius (99 degrees Fahrenheit), she weighs 7.3 kg (16 pounds 2 ounces), blood pressure is 90/45 mm Hg, respiratory rate 35/min, heart rate 120/min, oxygen saturation is 99 percent on room air. She begins to cry as you examine her, and this worsens her stridor. Visual inspection reveals nasal flaring, bilateral retractions, though no head bobbing or drooling is noted. Her neck is supple with no masses or lymphadenopathy detected. The patient’s skin is warm and dry, with no rashes or lesions. Her cardiopulmonary examination is normal, and her abdomen is soft without organomegaly. You are concerned about her appearance and order an x-ray of her throat (see image). Based on the information you have at this point, what is the most likely cause of this patient’s respiratory distress?