Nasal Fracture: This is the most likely diagnosis given the presentation of a possible broken nose. A nasal fracture, also known as a broken nose, occurs when the bone or cartilage in the nose is cracked or broken. This can result from a blow to the face, a fall, or other forms of trauma, which are common in adolescents.

Nasal Septal Hematoma: This condition involves a collection of blood in the nasal septum and can occur after trauma to the nose. It's a likely diagnosis because it can present similarly to a nasal fracture and also requires prompt treatment to prevent complications.
Soft Tissue Injury: Injuries to the soft tissues of the face, including lacerations, bruises, or swelling around the nose, are common after facial trauma and could be considered in the differential diagnosis.
Concussion or Mild Traumatic Brain Injury: Given that a blow significant enough to break the nose could also cause a concussion, this diagnosis should be considered, especially if there are symptoms such as headache, dizziness, or confusion.

Cerebrospinal Fluid (CSF) Leak: Although less common, a CSF leak can occur if the trauma causes a fracture in the cribriform plate or other parts of the skull base, leading to leakage of cerebrospinal fluid into the nasal passages. This condition requires immediate medical attention to prevent meningitis.
Basilar Skull Fracture: A fracture at the base of the skull can be life-threatening and may present with symptoms related to the nose, such as rhinorrhea (runny nose) that could be CSF, or other neurological symptoms.
Septic Arthritis or Osteomyelitis: Infections of the joints or bones, respectively, can occur as complications of trauma or fractures and require prompt antibiotic treatment to prevent long-term damage.

Wegener's Granulomatosis: A rare autoimmune disorder that can cause destruction of the nasal passages and sinuses, though it would be unusual to present acutely after trauma.
Nasal Tumor: Although rare, a tumor in the nasal cavity could potentially cause symptoms or be discovered incidentally after trauma to the nose.