Aplastic anemia

Aplastic anemia falls into the category of "anemias-in-which-the-cells-don't-look-weird" category. Anemias in this category can sometimes be difficult to diagnose (for example, in anemia of chronic disease, the cells are of normal size and shape...so it's dang hard to find anything weird to point you in a diagnostic direction).
In aplastic anemia, however, there's a giant clue staring you straight in the face: pancytopenia. Not only are the red cells decreased in number, but so are the white cells and platelets. That's because the marrow is basically empty. Check out the image above of a bone marrow biopsy from a patient with aplastic anemia. There's supposed to be hematopoietic tissue in there - but basically all you see are mature red cells and lymphocytes. All precursors are decreased: erythroblasts, granulocyte precursors, and megakaryocytes. So when you look at the blood, the mature cells in the blood are all decreased too.
Clinically, patients present with just what you'd expect: findings related to their cytopenias. Patients will usually be pale and fatigued (from the anemia), with bleeding and bruising (from the thrombocytopenia) and recurrent infections (from the leukopenia). Lots of things can cause aplastic anemia (drugs, viruses, pregnancy, Fanconi anemia), but many times, no cause is found.
Patients are usually treated with blood products as needed, and if the aplasia doesn't resolve, then drugs such as G-CSF and prednisone are given. Bone marrow transplant is successful in many patients, but because of the high morbidity and mortality of the procedure, it's used only as a last resort. For a serious hematologic disorder, the prognosis is relatively good: 3 year survival is on the order of 70%.