Substances such as cell-culture polystyrene can be used as negative (noncytotoxic) control.

• A. Test material
• B. Buffer
• C. Negative (noncytotoxic) control
• D. Positive (cytotoxic) control

Answer: (C)

In cytotoxicity testing, a negative control provides the baseline of non-toxic conditions so you can judge whether a test material harms cells. Substances like cell-culture polystyrene are designed to be biologically inert under standard culture conditions, so they should not affect cell viability and serve as this baseline. If the test material yields similar viability to the negative control, it is noncytotoxic under the tested conditions; a notable drop in viability would indicate cytotoxicity. The other roles differ: the test material is what you’re evaluating for toxicity, a buffer or vehicle serves as a medium rather than a baseline reference, and a positive control shows what cytotoxic damage looks like to confirm the assay detects it.