Which of the following can be an important indicator for diagnosing infectious mononucleosis?

The positive mono spot test is a significant indicator for diagnosing infectious mononucleosis, primarily because it detects heterophile antibodies in the patient's serum. These antibodies are typically produced in response to the Epstein-Barr virus (EBV), which is the most common cause of infectious mononucleosis. The serological test is quick and reliable, providing a strong indicator of recent infection with EBV, thus aiding in the diagnosis when symptoms such as fever, sore throat, and lymphadenopathy are present.

In assessing the context of other choices, the presence of a peripheral smear may provide additional information about the immune response, but it is not specific for infectious mononucleosis. Serum electrolytes and kidney function tests do not typically contribute to the diagnosis of this condition, as they measure metabolic and organ function rather than the presence of the specific viral infections related to mononucleosis. Therefore, the positive mono spot test remains the most relevant and effective diagnostic tool for this condition.