A 16-year-old girl exhibits symptoms of sore throat, fever, and tender cervical lymphadenopathy. What laboratory finding best supports her likely diagnosis?

The likelihood of the diagnosis for this 16-year-old girl, given her symptoms of sore throat, fever, and tender cervical lymphadenopathy, strongly suggests infectious mononucleosis, often caused by Epstein-Barr virus (EBV). The laboratory finding that best supports this diagnosis is atypical lymphocytosis.

Atypical lymphocytosis refers to an increase in atypical lymphocytes, which are often present in viral infections, particularly during infectious mononucleosis. These atypical lymphocytes are typically larger than regular lymphocytes and may have a more abundant cytoplasm. They are a hallmark of the immune response associated with this viral infection. Thus, the presence of atypical lymphocytes in the laboratory findings is a defining indicator for infectious mononucleosis.

While a high white blood cell count could occur in various infections, it is not specific to infectious mononucleosis. Similarly, a positive culture for Streptococcus group A would suggest streptococcal pharyngitis rather than mononucleosis. A positive monospot test is also commonly associated with infectious mononucleosis, but the presence of atypical lymphocytes is a more direct indicator of the viral nature of the illness, rather than just confirming an acute