Which statement about ultraviolet effects on tissue is true?

• A. Primary effect is photochemical
• B. Vitamin D production is the primary effect
• C. Both are primary effects
• D. None of the above

Answer: (A)

Ultraviolet light affects tissue mainly through photochemical reactions. The photons initiate chemical changes in skin molecules, especially DNA and other cellular components, leading to direct photochemical damage such as DNA lesions. This direct chemical alteration is the primary effect of UV exposure on tissue. Vitamin D production in the skin is a secondary biochemical outcome that occurs when UVB converts 7-dehydrocholesterol to previtamin D3; while important systemically, it is not the immediate tissue effect being described. Therefore, stating that the primary effect is photochemical correctly captures the main mechanism, and the other statements don’t describe the initial tissue response.