If a patient on penicillin develops a sore tongue, what test should be performed?

In the context of a patient on penicillin who develops a sore tongue, performing a fungal test is the most appropriate course of action. Penicillin can disrupt the normal balance of bacteria in the mouth, which may allow for an overgrowth of Candida, leading to oral thrush or candidiasis. A sore tongue can be a symptom of such a fungal infection.

Conducting a fungal test, such as a smear or culture, can help identify the presence of Candida species and confirm the diagnosis of oral thrush. This is essential for determining the correct treatment, which may involve antifungal medications.

The other options may not provide immediate or relevant information regarding the sore tongue. A blood test could help reveal systemic issues but is less specific for oral conditions. An allergy test would be appropriate if an allergic reaction to penicillin were suspected, but the symptoms described align more closely with a fungal infection. A culture test could also be beneficial, but it would typically be a part of the fungal testing process itself specifically aimed at identifying fungal pathogens.