Final answer:
To diagnose a sore tongue in a patient on penicillin, an oral swab for microscopic examination and fungal culture is recommended to check for oral thrush. A CBC may also provide useful information about the patient's immune status.
Step-by-step explanation:
If a patient has been on penicillin for 2 weeks and is now experiencing a sore tongue, it is important to consider the possibility of a fungal infection such as oral thrush (candidiasis), which can occur as a result of the disruption of the normal oral flora by antibiotic therapy. The recommended test for diagnosing oral thrush is an oral swab that is taken from the lesions present on the tongue. This sample is then subjected to a microscopic examination and possibly a fungal culture to confirm the presence of Candida species. It is also beneficial to consider a complete blood count (CBC) to assess the patient's immune status, as immune suppression may predispose to thrush. Additionally, it would be prudent to review the patient's medical history and any symptoms of allergy or adverse reactions to penicillin, as these medications can sometimes cause oral discomfort directly.