Viral Upper Respiratory Tract Infection (URTI): This is the most common cause of congestion and runny nose in infants, often caused by viruses such as rhinovirus, coronavirus, or adenovirus. The symptoms presented are typical for a viral URTI, and the age of the patient and the duration of symptoms support this diagnosis.

Allergic Rhinitis: Although less common in infants, allergic rhinitis can cause similar symptoms. However, it would typically be associated with other signs of allergy, such as eczema or a family history of allergies.
Vasomotor Rhinitis: This condition can cause nasal congestion and discharge due to non-allergic, non-infectious factors, but it's less common in infants.

Bacterial Sinusitis: Although less likely than viral URTI, bacterial sinusitis can occur, especially if symptoms worsen or persist beyond 7-10 days. It's crucial to consider this diagnosis due to the potential need for antibiotic treatment.
Influenza: Especially during flu season, influenza can cause severe illness in infants and requires prompt diagnosis and treatment.
Respiratory Syncytial Virus (RSV) Infection: RSV can cause mild to severe illness in young children, including bronchiolitis and pneumonia, making it a critical diagnosis not to miss.

Cystic Fibrosis: While rare, cystic fibrosis can present with recurrent respiratory infections and nasal congestion. It's essential to consider in infants with persistent or severe symptoms.
Choanal Atresia: A congenital condition where the nasal passage is blocked, which can cause respiratory distress and nasal congestion, especially noticeable in newborns but can be diagnosed later if symptoms persist.
Immunodeficiency: Certain immunodeficiencies can predispose infants to recurrent or severe infections, including those causing congestion and runny nose.