Which of the following signs is most characteristic of mononucleosis?

Marked fatigue is the most characteristic sign of mononucleosis, commonly caused by the Epstein-Barr virus. This condition often presents with profound tiredness that can persist for weeks or even months, significantly impacting the patient's quality of life. The fatigue associated with mononucleosis can be quite debilitating and is one of the hallmark symptoms that leads to the diagnosis of the illness.

While other symptoms may accompany mononucleosis, such as fever, sore throat, and swollen lymph nodes, the level of fatigue is particularly noteworthy and often serves as the primary complaint that prompts individuals to seek medical attention. This symptom distinguishes mononucleosis from other viral infections, where fatigue may be present but not to the same degree or duration.

The other options do not accurately represent the prominent symptoms of mononucleosis. Localized abdominal swelling could occur for various reasons but is not typical for mononucleosis. Severe joint pain is more characteristic of other conditions, and excessive sweating at night may be associated with infections but is not a defining feature of mononucleosis specifically. Thus, the profound fatigue provides a clear and specific indication of this virus's impact on health.