Viral upper respiratory tract infection (URTI): This is the most common cause of cough in children, often presenting with other symptoms like runny nose, sore throat, and fever. The high frequency of viral infections in this age group due to exposure in school or daycare settings makes this the most likely diagnosis.

Asthma: A common chronic condition in children that can cause cough, especially if it's persistent and accompanied by wheezing or shortness of breath. Asthma should be considered, especially if there's a family history or if the cough is worse at night or with exercise.
Allergic rhinitis: Allergies can cause a chronic cough, along with symptoms like sneezing, runny nose, and itchy eyes. This is more likely if the child has a history of allergies or if the cough is seasonal.
Pertussis (Whooping Cough): Although vaccination has reduced its incidence, pertussis can still occur, especially in areas with low vaccination rates. It's characterized by a distinctive whoop sound during coughing fits, which can be severe.

Pneumonia: While less common than URTI, pneumonia is a serious condition that requires prompt treatment. Symptoms can include high fever, difficulty breathing, and cough. It's crucial to consider pneumonia, especially if the child appears severely ill or has risk factors like underlying health conditions.
Foreign body aspiration: This is an emergency that can cause sudden onset of cough and respiratory distress. It's more common in younger children but should be considered in any child with a sudden, unexplained cough.
Tuberculosis (TB): Although rare in many areas, TB is a critical diagnosis not to miss due to its potential severity and the need for specific treatment. It might be considered if the child has been exposed to someone with TB, has traveled to or lived in areas with high TB prevalence, or has symptoms like weight loss, night sweats, and a chronic cough.

Cystic Fibrosis: A genetic disorder that can cause chronic cough, among other symptoms like recurrent lung infections and poor growth. It's rare but should be considered in a child with persistent respiratory symptoms and other signs suggestive of cystic fibrosis.
Bronchiectasis: A condition characterized by damaged airways, leading to chronic cough and recurrent infections. It's less common in children but might be considered in those with a history of severe infections or underlying conditions that could lead to airway damage.
Habit cough: A rare condition where the cough is psychological in origin, often seen in older children. It's usually a dry, honking cough that occurs only when the child is awake and often resolves with reassurance and time.