All of the following are diagnostic tests for leukemia EXCEPT which one?

The correct choice is the fecal occult blood test, as it is not a diagnostic test typically used for leukemia. This test is primarily designed to detect hidden (occult) blood in the stool, which can be indicative of gastrointestinal disorders such as colorectal cancer, but it does not provide relevant information about blood cancers like leukemia.

In contrast, a bone marrow biopsy is a key diagnostic procedure for leukemia. It involves taking a sample of the bone marrow to examine the quality and quantity of blood cells being produced and to look for abnormal cells that are characteristic of leukemia.

The differential white blood count is another critical test used in the diagnosis of leukemia. It measures the different types of cells in the blood, notably white blood cells, and can reveal abnormalities typical of various leukemia types.

A lumbar puncture, while not as common, can also be used in specific scenarios to check for leukemia cells in the cerebrospinal fluid, especially if there is a concern about central nervous system involvement.

Hence, the fecal occult blood test stands out as unrelated to the diagnosis of leukemia, making it the correct answer.