Infectious Mononucleosis (Mono): This condition, caused by Epstein-Barr virus (EBV), is characterized by pharyngitis, lymphadenopathy, and often a strawberry-like appearance of the tongue. The negative strep test helps to rule out streptococcal pharyngitis, making mono a leading consideration.

Viral Pharyngitis: Various viruses can cause pharyngitis, and some, like adenovirus, can present with lymphadenopathy and a strawberry tongue. The absence of a positive strep test and the presence of these symptoms make viral pharyngitis a plausible diagnosis.
Scarlet Fever: Although the strep test is negative, scarlet fever (caused by Group A beta-hemolytic streptococcus) can sometimes yield false-negative results, especially if the swab is not taken correctly or if the patient has been on antibiotics. Scarlet fever is known for causing a strawberry tongue and lymphadenopathy.

Lemierre's Syndrome: This is a rare but potentially life-threatening condition that starts with pharyngitis and can progress to septicemia and thrombophlebitis of the internal jugular vein. It's crucial to consider this diagnosis due to its severity, even though it's less common.
Diphtheria: Although rare in areas with good vaccination coverage, diphtheria can cause severe pharyngitis and lymphadenopathy. A strawberry tongue is not the most common presentation, but the disease's potential severity warrants its consideration.

Kawasaki Disease: This condition primarily affects children and is characterized by fever, rash, swelling of hands and feet, conjunctivitis, and lymphadenopathy. A strawberry tongue can be part of the presentation. While it's a rare cause of pharyngitis, its potential for cardiac complications makes it worth considering in the differential diagnosis.
Toxic Shock Syndrome: Caused by bacterial toxins, this condition can present with severe pharyngitis, lymphadenopathy, and a strawberry tongue, among other symptoms. It's rare but life-threatening and requires prompt recognition and treatment.