Which of the following is NOT a common cause of protein stains?

Water is not a common cause of protein stains because it does not contain organic material that could lead to staining. Protein stains typically arise from substances that contain proteins, which can bind to fibers in upholstery and fabrics. Blood, milk, and urine are all rich in proteins: blood contains hemoglobin and other proteins, milk is a dairy product high in casein and whey proteins, and urine contains urea and other organic nitrogenous compounds. These substances can all result in staining when they come into contact with fabrics. In contrast, water is a solvent that can help remove stains rather than create them, so it does not contribute to protein stains.