Final answer:
Deficiency in the enzyme thiopurine S-methyltransferase (TPMT) can lead to severe neutropenia when treated with 6-MP, due to the buildup of toxic metabolites affecting DNA synthesis and white blood cell proliferation.
Step-by-step explanation:
The enzyme whose deficiency can cause severe neutropenia when a patient is treated with 6-MP (Mercaptopurine) is thiopurine S-methyltransferase (TPMT). TPMT metabolizes thiopurines, such as 6-MP, into inactive metabolites. When TPMT activity is deficient, patients can have excessive levels of active 6-MP metabolites leading to toxicity, which includes neutropenia, a condition marked by an abnormally low count of neutrophils, a type of white blood cell important for fighting infection. This can occur because 6-MP is an analogue of the purine adenine and affects DNA synthesis, which is necessary for the proliferation of cells, including neutrophils.