First, this isn't "discrimination". This making you earn the right to vote. No one can vote until they pass. And its completely in your control - no one is suddenly excluded from earning the ability to vote (except for those we already deny it to for one reason or another) because of something they can't control. Here is the line - if you want to vote, step over it. If you don't, don't whine about being unable to vote - everyone had the same chance. Ignorance is a choice, not a given.
Second, don't get all happy for passing this part alone. The real test would be 150 - 200 questions long, and go into a lot more detail. No questions are optional. While designing each year's test could be a challenge, I suppose it could be done by a small committee, or several universities. Things like:
-What is the difference between Medicaid and Medicare? (Medicaid is the state-level system, Medicare is the federal version)
-What Amendment is generally referred to as "State's Rights"? (The 10th)
-Name 3 Republican Presidents before President George H.W. Bush.
-Name 3 Democratic Presidents before President William Clinton.
-The United States is a ? (Circle One)
Democracy or Republic (Republic)
-Name the 4 branches of the US Military and the former branch that is now part of the Department of Transportation. (Army, Navy, Air Force, Marines and Coast Guard)
-Who is the Speaker of the House? (Nancy Pellousi)
-Which President is most responsible for the Interstate Highway system? (President Eisenhower)
Math and logic ones are a little harder to come up with, so I let those be right now.
Further, this is just common sense. You wouldn't let these people work on your car - why would you let them pick your leadership? And this kind of knowledge and ability is essential to being a responsible citizen. To not have knowledge of your country, its past, and the general ideas of the canidates, officials and leaders is a recipe for disaster.
Starship Troopers, which has already been mentioned here several times, had it right. When everyone has the right to vote, it'll work for a while, but you'll eventually wind up with chaos.
If America's recent past is any indication, we're staring over the edge as we speak.