Viral Pharyngitis: This is the most common cause of sore throat in children and adolescents, often caused by viruses such as adenovirus, rhinovirus, or coronavirus (given the COVID-19 test is negative, another coronavirus or virus is likely). The absence of fever and other symptoms supports this diagnosis.

Streptococcal Pharyngitis (Strep Throat): Although less common than viral pharyngitis, strep throat is a significant cause of sore throat in this age group, especially if the sore throat is moderately severe. It's crucial to consider this diagnosis due to the potential for complications if left untreated.
Infectious Mononucleosis (Mono): Caused by Epstein-Barr virus (EBV), mono can present with sore throat, although it often includes other symptoms like fever, fatigue, and lymphadenopathy. The absence of fever doesn't rule out mono, especially in early stages.
Allergic Rhinitis or Postnasal Drip: These conditions can cause a sore throat due to the constant drip of mucus from the nose down the back of the throat, irritating it.

Lemierre's Syndrome: A rare but potentially life-threatening condition that starts with a sore throat, followed by neck pain and fever, caused by Fusobacterium necrophorum. It's essential to consider this diagnosis due to its severe complications, including sepsis and thrombophlebitis.
Peritonsillar Abscess: A collection of pus behind the tonsils that can cause severe sore throat, difficulty swallowing, fever, and sometimes a muffled voice. It requires prompt medical attention.
Epiglottitis: Inflammation of the epiglottis, which can be life-threatening due to the risk of airway obstruction. Although more common in younger children, it should be considered in any patient with a severe sore throat, especially if there are symptoms like difficulty swallowing or shortness of breath.

Diphtheria: A bacterial infection affecting the throat and salivary glands, characterized by a sore throat and a greyish membrane covering the tonsils and throat. It's rare in areas with high vaccination coverage but should be considered in unvaccinated individuals or those traveling from areas where diphtheria is common.
Tuberculosis (TB): Although rare, TB can cause a chronic sore throat among other symptoms. It's more likely in individuals with exposure to TB or those with weakened immune systems.
Neoplastic Conditions: Such as lymphoma or other cancers that can present with a sore throat, although these are exceedingly rare in children and would typically be accompanied by other significant symptoms.