A 20-year-old with sore throat and fever; labs show neutrophils 50 (L), lymphocytes 45 (H), negative Strep, negative Mono. What is the most likely diagnosis?

A 20-year-old with sore throat and fever; labs show neutrophils 50 (L), lymphocytes 45 (H), negative Strep, negative Mono. What is the most likely diagnosis?

• A. Group A Streptococcus
• B. Bacterial pharyngitis
• C. Viral pharyngitis
• D. Mononucleosis

Answer: (C)

The key idea is how the white blood cell pattern helps distinguish viral from bacterial throat infections. Bacterial pharyngitis often shows a neutrophil-dominant response, sometimes with leukocytosis and a positive strep test. Viral pharyngitis, by contrast, tends to have a normal or low overall WBC with relatively more lymphocytes (lymphocytosis) and a negative strep test. Here, fever with sore throat and a differential showing neutrophils around 50% and lymphocytes around 45% (a mild relative lymphocytosis), plus negative tests for Strep and mononucleosis, fits a viral pattern more than a bacterial one. The absence of neutrophil predominance argues against bacterial pharyngitis, and the mono test being negative makes mononucleosis less likely despite the age. So the most likely diagnosis is viral pharyngitis, managed with supportive care rather than antibiotics.

The key idea is how the white blood cell pattern helps distinguish viral from bacterial throat infections. Bacterial pharyngitis often shows a neutrophil-dominant response, sometimes with leukocytosis and a positive strep test. Viral pharyngitis, by contrast, tends to have a normal or low overall WBC with relatively more lymphocytes (lymphocytosis) and a negative strep test.

Here, fever with sore throat and a differential showing neutrophils around 50% and lymphocytes around 45% (a mild relative lymphocytosis), plus negative tests for Strep and mononucleosis, fits a viral pattern more than a bacterial one. The absence of neutrophil predominance argues against bacterial pharyngitis, and the mono test being negative makes mononucleosis less likely despite the age. So the most likely diagnosis is viral pharyngitis, managed with supportive care rather than antibiotics.