A 30 years old male reported with chief complaint of constant radiating, gnawing pain that is intensified by eating spicy foods and chewing. He also reported a "metallic" foul taste and an excessive amount of "pasty"saliva. A general examination revealed fever and increased pulse rate. Intra-oral examination showed punched-out, crater like depressions at the crest of the interdental gingival papillae, covered by a gray pseudomembranous slough, in the upper anterior teeth. The most likely infection is