Chaining selectors in jQuery

I'm a guy used to mootools' way of chaining selectors, and I can't seem to find anywhere how to do the same in jQuery.Suppose I have a select element in the selectObj variable. What I need is to get the last option in that select.In mootools I would have done something like:

var option = $(selectObj).getElement('nth-child(last)')

Can I do something similar, or what is the way of getting that last option in jQuery?

PS. I know about the parent > child selector, but I can't really use it because I don't know what selector has been used to get the select. I only have the resulting element.