Which of the following compounds is NOT a type of tetracycline?

Tetracyclines are a class of broad-spectrum antibiotics that include several compounds, with tetracycline, doxycycline, and minocycline being key members of this group. These antibiotics work by inhibiting protein synthesis in bacteria, making them effective against a range of infections.

Penicillin, on the other hand, belongs to a different class of antibiotics known as β-lactams. It operates through a different mechanism, primarily by disrupting bacterial cell wall synthesis. Since penicillin does not share the same structure or function as the tetracyclines, it cannot be classified as a type of tetracycline.

Thus, when identifying a compound that is not a type of tetracycline, penicillin is the correct choice due to its distinct classification and mode of action compared to the other listed antibiotics, which are all tetracyclines.