Arc tool gives me "elbows"

I select the Arc tool. I click and drag and what Illustrator gives me is a "rounded" right-angle, or "elbow". Or a lopsided arc. I open the Arc Segment Tool Options and adjust the Concave/Convex slider but this only changes the depth of the "elbow", not its shape.

How do I get an arc similar to a 1/4 circle?

(I used to draw these but now I can't.) I'm sure I'm overlooking something.

Thanks,

iX

Illustrator CS 5