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 –
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.
Acute Epiglottitis
Laryngotracheobronchitis
Foreign body aspiration
Retropharyngeal abscess