Which of the following is classified as an antinfective?

Tetracycline is classified as an antibiotic, which falls under the category of anti-infectives. It is commonly used to treat a variety of bacterial infections by inhibiting protein synthesis in bacteria, effectively preventing their growth and reproduction. This mechanism makes it a valuable tool in the management of infections caused by susceptible organisms.

The other choices provided are not classified as anti-infectives. Metaproterenol is a bronchodilator primarily used to treat asthma and other breathing disorders, while Solumedrol (methylprednisolone) is a corticosteroid used for its anti-inflammatory effects in various conditions. Theophylline is a medication used to relax the muscles of the airways and improve breathing, but it does not have antimicrobial properties. Thus, Tetracycline stands out as the only true anti-infective agent among the listed options.