Which side effect of phenothiazines involves sensitivity to light?

The correct answer is photosensitivity, which is a known side effect of phenothiazines. Photosensitivity refers to an increased sensitivity to sunlight, leading to an exaggerated skin reaction when exposed to ultraviolet light. This can result in conditions such as sunburn or rashes, even with minimal exposure. Since phenothiazines can alter the skin's response to sunlight, patients taking these medications are often advised to take precautions when spending time outdoors, such as using sunscreen or wearing protective clothing.

Drowsiness is indeed a common side effect of phenothiazines, but it does not relate to sensitivity to light. Granulocytosis is a more severe condition involving a decrease in white blood cells, which can lead to increased infection risk, but it is unrelated to light sensitivity. Extrapyramidal syndrome includes movement disorders such as tremors or rigidity, which are also not connected to photosensitivity. Thus, among the choices provided, photosensitivity stands out as the only option directly linked to light sensitivity due to the pharmacological effects of phenothiazines.