Which microorganism lacks its own metabolic pathway and can only reproduce inside a host cell?

The correct answer is a virus, as viruses are unique among microorganisms due to their reliance on host cells for reproduction and metabolic processes. Viruses consist of genetic material (either DNA or RNA) enclosed in a protein coat and sometimes a lipid envelope. They do not possess the cellular structures necessary to carry out metabolic activities or energy production independently. Instead, viruses infect host cells and hijack the cellular mechanisms of the host to replicate their genetic material and produce new viral particles. This dependence on a host cell for reproduction is a defining characteristic of viruses, distinguishing them from other types of microorganisms like bacteria, fungi, and protozoa, which have their own metabolic pathways and can reproduce independently of a host.