Which one of the following oral soft tissue diseases is most likely to occur after a patient has taken a long course of broad-spectrum antibiotics?

Oral candidiasis, commonly known as oral thrush, is a fungal infection caused by the overgrowth of Candida species, particularly Candida albicans. When a patient has undergone a long course of broad-spectrum antibiotics, these medications can disrupt the normal balance of flora in the mouth and throughout the body. Antibiotics are designed to eliminate harmful bacteria but often do not discriminate, leading to the reduction of beneficial bacteria that typically keep Candida in check.

As a result, the suppression of these protective bacteria allows for the opportunistic growth of Candida, leading to oral candidiasis. Symptoms may include white patches on the tongue, inner cheeks, and other areas of the mouth, along with possible discomfort or pain.

In contrast to this, herpes simplex and herpetiform aphthous ulcers are viral and inflammatory conditions respectively and do not typically have a direct association with antibiotic use. Leukoplakia is a condition characterized by white patches in the oral cavity which can result from various factors, including irritation or tobacco use, but is not directly related to antibiotic use as a trigger. Thus, oral candidiasis is the most likely condition to develop following prolonged antibiotic therapy due to the alteration of the oral microbiome.