Final answer:
Viral infections are the most common nonmalignant cause of lymphocytosis in adolescence and adulthood, whereas lymphoma represents a malignant cause of elevated lymphocyte counts.
Step-by-step explanation:
The most common nonmalignant cause of lymphocytosis in adolescence and adulthood is typically due to viral infections. Lymphocytosis signifies an increased number of lymphocytes in the bloodstream, which is often a response to infections, particularly viral ones. While certain types of cancer, like lymphoma, can also cause elevated lymphocyte counts, in the absence of malignancy, infections remain the primary culprit. In many cases, such as the scenario described where antibiotics are prescribed, a bacterial origin is considered but viral causes are more prevalent overall.