What finding would support a diagnosis of infectious mononucleosis (IM)?

The key finding that supports a diagnosis of infectious mononucleosis is cervical lymph node enlargement and tenderness. Infectious mononucleosis, often caused by the Epstein-Barr virus, typically presents with symptoms that include fatigue, fever, sore throat, and notably, swollen lymph nodes, particularly in the cervix area. The enlargement and tenderness of these lymph nodes occur due to the body's immune response to the viral infection, making this symptom a classic indicator of the condition.

In contrast, while abdominal pain may occur in some patients due to splenic or hepatic involvement, it is not a direct hallmark of infectious mononucleosis. Similarly, a systolic heart murmur can result from various conditions but is not associated specifically with IM. Conjunctival injection, which refers to redness in the eyes, is also not a defining characteristic of IM, as it typically relates more to allergic reactions or infections that directly impact the eyes. Thus, cervical lymph node enlargement and tenderness distinctly support the diagnosis of infectious mononucleosis.