Final answer:
Metronidazole is correctly used to treat necrotizing ulcerative gingivitis (NUG), commonly referred to as trench mouth, a severe form of periodontitis. It is not typically used for denture stomatitis, recurrent aphthous ulcers, or primary herpetic gingivostomatitis.
Step-by-step explanation:
Metronidazole is an antibacterial and antiprotozoan medication that can be used to treat various infections. When considering the options presented in the question, the correct usage of metronidazole is for the treatment of necrotizing ulcerative gingivitis (NUG), also known as trench mouth. This condition involves severe periodontitis characterized by gum erosion, ulcerations, significant pain, and bad breath, often associated with a weakened immune system.
Conditions such as denture stomatitis, recurrent aphthous ulcers, and primary herpetic gingivostomatitis are not typically treated with metronidazole. Denture stomatitis is often related to fungal infections, namely candidiasis, and requires antifungal treatment rather than an antibacterial like metronidazole. Recurrent aphthous ulcers generally do not have a bacterial cause and are not treated with antibiotics. Lastly, primary herpetic gingivostomatitis is caused by herpes simplex virus and requires antiviral treatment rather than antibiotics like metronidazole.