Where do blood cells come from?

Well, conferences are over for the time being, so I'm now able to post daily once again. Here's today's question:

Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the: (and then it goes on to list some places).

Ok, this is a relatively simple question that tests your knowledge of some basic anatomy and physiology. First things first: I've noticed that these tests just love making questions seem more complex than they really are. Take this one, for example. The very first line talks about high levels of radiation. I don't know about you, but I studied very little radiation in my biology classes, so when I first read the question, I got a bit worried about what I'm supposed to know. The question is misleadingly complex. If you just take out the radiation bits, you get a question that goes something like: "Damage to what part causes anemia?" That is much, much easier to answer! So I'm just going to skip the explanation about radiation and its dangers, and jump right into the meat of the question: what is anemia?

Anemia is a deficiency of hemoglobin or red blood cells. Hemoglobin deficiency lowers the blood cell's ability to capture and transport oxygen throughout the body (bad, yes?) and a lowered red blood cell count causes basically the same thing. Either way, anemia is bad. Your tissues need oxygen, and the red blood cells are there to get it to them. Without red blood cells, you die. A lot.

Now, I'm sure some of you have been told you need to take iron to prevent or treat mild anemia. This is true, but don't let it confuse you when you go to answer the question. Iron is a precursor to hemoglobin. The most common form of anemia is lack of hemoglobin, so taking iron supplements allows your body to produce more hemoglobin and therefore transport more oxygen. Lack of oxygen can cause lethargy, hence the tired feeling associated with anemia.

This question, however, is referring to the other form of anemia: lack of red blood cells. How do I know? I looked at the answer list! Here's the question again (this time with the answer list present):

Exposure to high levels of radiation in humans has been demonstrated to cause anemia. The most likely explanation for this is that the radiation damages the:
A) Blood vessels
B) Spleen
C) Liver
D) Thymus
E) Bone marrow

I advocate answering the question before you look at the answers, but sometimes the first answer you come up with isn't listed. Here was my thought process as I read this question: "Well, anemia is caused by lack of hemoglobin or red blood cells, so radiation must attack the red blood cells themselves." As you can see, this answer isn't listed. If your top answer isn't there, go through the rest of the answers and see which one makes the most sense.

Blood vessels. While damage to the blood vessels could cause blood leakage into various body cavities and eventually cause anemia due to lack of blood cells circulating, anemia isn't the first worry. I would be much more worried about internal bleeding, which would probably present as pain or death. Tiny amounts of internal bleeding may cause anemia, but that would mean only tiny amounts of radiation damage, and that isn't likely unless the radiation was controlled in some way (as in radiation therapy). I disregard this one right off the bat.

Spleen. Anyone who has studied the circulatory system knows that the spleen is involved. The spleen filters worn out red blood cells and sends them to the liver for processing. It also holds a small amount of blood in reserve for times when you need that extra burst of oxygen--like exercising or hiking at high altitudes. This makes your blood system more efficient. However, you can live without this little extra burst without any ill effects. Lacking a spleen doesn't cause anemia. It just like living without a savings account--not the most comfortable way to live, but it doesn't mean your checking account has any less money than it would have otherwise.

Liver. The liver does bunches and bunches of things that you don't need to know about at the moment. One major job is the break down of red blood cells and the recycling of hemoglobin. The liver breaks down the worn out red blood cells and gets rid of the excess material via billirubin. Liver damage would cause major problems in a person, but wouldn't cause anemia.

Thymus. The thymus gland is a place where certain white blood cells go to mature. Don't worry, I'm sure there's a question about white blood cells coming up that I can use to address this issue. Just know that it doesn't cause anemia.

Bone marrow. Ah, we've found it. Bone marrow is what gives rise to all the blood cells circulating in your blood stream. Immature blood cells are formed in the bone marrow, then travel to a variety of places to mature. If the bone marrow gets damaged, it no longer can produce blood cells, which will result in a lowered red blood cell count and eventually anemia. Going back to that bank account example, while the spleen is like your savings account, the bone marrow is like your job. While you can live just fine without a stash of money somewhere, if your income gets cut off then your screwed. Bank account anemia!

So, the answer to this question is "E" bone marrow. Yay!