a 16-year-old girl with acute onset respiratory distress

a 16-year-old girl with an intellectual disability (known case of down syndrome) arrived in the emergency eepartment with complaints of severe breathlessness, bloody salivation (bright red blood or clots), and difficulty in speaking and swallowing of liquids and solids. the patient gradually developed progressive bloody salivation and hoarseness, never had any history of trauma to the head and neck and respiratory problems, and was symptomatic from the previous 48 h. furthermore, the patient had a positive history of peptic ulcer, chronic consumption of non-steroidal anti-inflammatory drugs, and traveled out of town and drank water from a well in the mentioned period. on admission, the patient had a respiratory rate of 17 per min, pulse rate of 89 per min, blood pressure of 120/80 mmhg, 90% o2 saturation in room air, and 38°c axillary temperature. the only positive finding on physical examination was inspiratory stridor. the auscultation of lung and heart sounds was normal. digital rectal examination revealed brown feces. throat examination was not possible owing to lack of patient cooperation. after initial assessment and essential consideration, electrocardiography (ecg) and imaging was performed. the ecg showed normal sinus rhythm, and analysis of arterial blood gas revealed the following: ph = 7. 35, pa-co2 = 39 mmhg, hco3 = 24 meq/l, pao2 = 89 mmhg, and o2 saturation = 92%. all other laboratory data, including complete blood counts (cbc), urine analysis, hepatic and renal function tests, and coagulation profile were in the normal range.   what is your diagnosis?