The most accurate description from the provided options is:
"The median rural income is less than the median urban income."
This statement reflects a well-documented trend in income inequality, where urban areas typically have higher median incomes compared to rural areas in the U.S. The other statements are generally misleading or incorrect. For instance, while the U.S. has significant income inequality, it is not necessarily the most unequal country in the world when compared to other nations. Additionally, while income gaps exist between genders and ethnicities, the statement about the absence of gaps between different ethnicities is incorrect.