The ability of a chemical agent to kill which organism defines its classification as a high-level disinfectant?

Correct answer: D. Mycobacterium tuberculosis

• A. ❌ E. coli is readily killed and is not a benchmark organism.
• B. ❌ S. aureus is a vegetative bacterium killed even by low-level disinfectants.
• C. ❌ Fungi are less resistant than mycobacteria and do not set the standard.
• D. ✅ CORRECT — Tuberculocidal activity is the benchmark. Mycobacteria have a waxy, lipid-rich cell wall that makes them among the most resistant vegetative organisms, so an agent that kills M. tuberculosis is presumed to kill less resistant vegetative bacteria, most viruses and fungi. Intermediate-level disinfectants are tuberculocidal; low-level ones are not.

Note: Resistance ladder (most → least): prions > bacterial spores > mycobacteria > non-enveloped viruses > fungi > vegetative bacteria > enveloped viruses.