3. Which of these organs is directly involved in synthesis of red blood cells in an adult?
A. Liver
B. Spleen
C. Bone marrow
D. Lymph nodes

*Answer*: C.Bone marrow
*Explanation*: In the human adult, the bone marrow produces all of the red blood cells, 60–70 percent of the white cells (i.e., the granulocytes), and all of the platelets.

Why the answer not A,B&D:

The lymphatic tissues, particularly the thymus, the spleen, and the lymph nodes, produce the lymphocytes (comprising 20–30 percent of the white cells). The reticuloendothelial tissues of the spleen, liver, lymph nodes, and other organs produce the monocytes (4–8 percent of the white cells). The platelets, which are small cellular fragments rather than complete cells, are formed from bits of the cytoplasm of the giant cells (megakaryocytes) of the bone marrow.