Which of the following changes does NOT occur when the skin is exposed to ultraviolet rays?

When skin is exposed to ultraviolet (UV) rays, various physiological changes occur as a protective response to UV radiation. Among these changes, increased melanin production is triggered to absorb and mitigate UV damage, which leads to the darkening of the skin — a process known as tanning. Additionally, the skin may undergo structural damage over time due to UV exposure, resulting in the formation of wrinkles and the general aging of the skin.

However, the thinning of the horny layer, also known as the stratum corneum, is not a typical response to UV exposure. Instead, UV exposure can cause thickening of the skin in certain areas as a protective measure against damage. Over time, though, excessive UV exposure could lead to a breakdown of collagen and elastin, contributing to wrinkles but not thinning of the outermost skin layer.

Thus, the option regarding the thinning of the horny layer accurately reflects a change that does not occur as a direct result of UV exposure, making it the correct answer.