A tetrose is a monosaccharide with 4 carbon atoms. They have either an aldehyde functional group in position 1 ( aldotetroses ) or a ketone functional group in position 2 ( ketotetroses ).  
Draw the Fischer projection for the D-2-ketotetrose. Switch between
an aldose and a ketose by clicking on \"switch carbonyl group.\"
Add or delete carbon atoms using the add (\" \") or delete (\"x\")
buttons. Clicking on a blue box once adds a hydrogen atom (H).
Clicking on a blue box again toggles between H and OH.