Which products should be strictly avoided for use on pregnant or nursing clients?

The correct answer encompasses the idea that all listed products—salicylic acid, vitamin A, and TCA—should be avoided for use on pregnant or nursing clients due to safety concerns.

Salicylic acid, particularly in high concentrations commonly found in some acne treatments, can be absorbed into the bloodstream and has associations with potential risks for fetal development.

Vitamin A, especially in its retinoid form, is known for its effectiveness in treating acne but poses significant risks during pregnancy and breastfeeding, as excessive vitamin A can lead to teratogenic effects, potentially causing birth defects.

TCA, or trichloroacetic acid, is a chemical peel agent that is also not recommended due to the potential for systemic absorption and the risk it poses to a developing fetus or nursing infant.

These products all carry potential risks due to their active ingredients' ability to affect hormonal balance or fetal development directly. Therefore, it is imperative for practitioners to avoid using any of these on pregnant or nursing clients to ensure the safety of both the mother and child.