What type of skin discoloration often appears as irregular blotches and can vary in size?

The correct answer is a stain. Stains represent skin discolorations that often manifest as irregular blotches and can vary significantly in size. These are typically caused by an accumulation of pigment or alterations in blood flow, leading to uneven coloring of the skin.

In contrast, scars result from the healing process following an injury, and while they can also have variations in size, they usually have a more defined appearance where the tissue has healed, compared to the irregular nature of a stain. Spots are generally smaller, more uniform in size, and may refer to localized areas of discoloration, while patches usually describe larger areas of discolored skin but tend to be more uniform in shape than stains. Therefore, a stain is the most fitting term for irregular, variably sized skin discolorations.