Here are your options with CSS alone using some advanced selectors.
option 1
As there is currently only 1 itemprop div within the #wrapper, just select the div using the itemprop attribute - working fiddle
#wrapper div[itemprop="description"] { // your style }
option 2
This will select the last child element provided it's a div
#wrapper div:last-of-type { // your style }
option 3
As others have pointed out you can use :nth-child() selector. I wouldn't recommend this if the number of divs within #wrapper will vary though.
#wrapper div:nth-child(3) {
// your style
}
option 4
If you're going to have multiple itemprop divs inside the wrapper you can cancel out styling by selecting siblings. However, this will only work on the itemprop divs within the #wrapper.
#wrapper div[itemprop="description"] { // your style }
#wrapper div[itemprop="description"] ~ div[itemprop="description"] { // cancel out styling }