Which of the following drugs is known to increase white blood cell count?

Lithium is recognized for its ability to increase white blood cell count, particularly in the context of its use as a mood stabilizer in treating bipolar disorder. This effect can be beneficial as it aids in mood regulation and has implications for patients who may experience leukopenia, or low white blood cell counts. The mechanism of action for lithium's effect on white blood cells may involve modulation of bone marrow function and stimulation of leukocyte production.

The other drugs mentioned do not have the same significant effect on white blood cell counts. Fluoxetine, an antidepressant, primarily works on serotonin pathways and does not directly influence white blood cell production. Ibuprofen, a nonsteroidal anti-inflammatory drug (NSAID), is primarily used for pain relief and inflammation but does not impact white blood cell counts in a meaningful way. Metformin, used for managing type 2 diabetes, also does not have a notable effect on increasing white blood cell levels. Therefore, lithium stands out as the drug associated with an increase in white blood cell count, making it the correct answer in this context.