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Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
A child with three days history of upper respiratory tract infection presents with stridor, which decreases on lying down position. What is the most probable diagnosis –
assistant
The correct answer is  This child has : -

3 days history of upper respiratory tract infection.
Followed by stridor

These features suggest the diagnosis of croup.
Clinical manifestations of Croup 

Most patients have an upper respiratory tract infection with some combination of -
Rhinorrhea               
A mild cough     
Pharyngitis 
Low-grade fever         
After 1-3 days signs and symptoms of upper respiratory tract obstruction become apparent -
Barking cough
Hoarseness  
Inspiratory stridor                           
Symptoms are characteristically worse at night.
Symptoms often recur with decreasing intensity for several days and resolve completely in one week.
Agitation and crying aggravate the symptoms.
The child may prefer to sit up in bed or be held upright.


Chest X-ray

Characteristic narrowing of the subglottic region known as the steeple sign.