Lysozyme as an extracellular antibacterial enzyme

Steps:

• Locate lysozyme in tears, saliva, milk, and mucus, which are extracellular secretions.
• Identify its function: hydrolyzes peptidoglycan in bacterial cell walls, providing antibacterial defense.
• Classify it as a protein enzyme, not an antibody, based on its enzymatic action.
• Rule out intracellular location since it's secreted outside cells.

Why C is correct:

• Lysozyme is defined as an extracellular enzyme that degrades bacterial peptidoglycan, acting as a non-specific antibacterial protein in mucosal secretions.

Why the others are wrong:

• A: Lysozyme is an enzyme, not an antibody, which are specific immunoglobulins.
• B: Identical to A; lysozyme lacks antibody structure and antigen-binding function.
• D: Lysozyme operates extracellularly in body fluids, not inside cells.

Final answer: C