Deviated Nasal Septum: This condition is a common cause of unilateral nasal congestion and can occur due to trauma, genetic predisposition, or developmental issues. The deviation of the nasal septum can lead to turbulent airflow, causing congestion on the affected side.

Nasal Polyps: These are growths that can occur inside the nasal passages and sinuses, leading to chronic congestion, loss of smell, and discharge. They can be associated with allergies, asthma, or infection.
Chronic Sinusitis: Inflammation of the sinuses can cause persistent nasal congestion, facial pain, and discharge. This condition can be due to infection, allergies, or anatomical issues.
Turbinate Bone Hypertrophy: The turbinate bones in the nose can become enlarged, leading to obstruction of airflow and congestion. This can be a primary condition or secondary to other nasal issues.

Nasal Tumors (Benign or Malignant): Although rare, tumors in the nasal cavity or sinuses can cause unilateral congestion, bleeding, or loss of smell. Early detection is crucial for effective treatment.
Invasive Fungal Sinusitis: This is a serious condition that can occur in immunocompromised individuals, leading to destruction of the nasal and sinus tissues. It requires prompt diagnosis and treatment to prevent severe complications.
Granulomatosis with Polyangiitis (GPA): Formerly known as Wegener's granulomatosis, GPA is a systemic vasculitis that can affect the nasal passages and sinuses, causing congestion, bleeding, and destruction of tissue.

Nasal Foreign Body: Although more common in children, a foreign body in the nasal cavity can cause unilateral congestion and discharge in adults as well.
Choanal Atresia: A congenital condition where there is a blockage of the choana (the passageway connecting the nose to the throat), which can cause unilateral nasal congestion.
Sarcoidosis: A systemic disease that can affect the nasal passages and sinuses, leading to congestion, granulomas, and other symptoms.