Leukopenia can occur in all of the following conditions EXCEPT:

Leukopenia, a condition characterized by a decreased white blood cell count, is commonly observed in situations where there is a reduction in the bone marrow's ability to produce white blood cells or an increased destruction of these cells.

In the context of the conditions listed, chemotherapy is known to damage the bone marrow, leading to reduced production of leukocytes, which can result in leukopenia. Similarly, viral infections can cause leukopenia by either affecting bone marrow function or through mechanisms that lead to increased peripheral destruction of white cells. Bone marrow infiltration, whether due to malignancies or other diseases, can disrupt normal hematopoiesis, leading to decreased white blood cell production and therefore leukopenia.

In contrast, a leukemoid reaction is a physiological response to an infection or significant stress to the body that results in an increased production of white blood cells, often resembling leukemia. This condition typically does not lead to leukopenia; rather, it results in leukocytosis, or increased white blood cell counts, as the body attempts to respond to the underlying stressor.

Hence, the correct answer indicates a condition in which leukopenia would not occur, thus highlighting the unique nature of leukemoid reactions in comparison to other conditions