Which option represents a potential synergistic risk when combined with doxorubicin?

The combination of doxorubicin with cyclophosphamide represents a potential synergistic risk because both agents are known to have myelosuppressive effects. When these two chemotherapy drugs are used together, there is an increased likelihood of exacerbating bone marrow suppression, which can lead to greater risks of neutropenia, thrombocytopenia, and anemia. This intensification of toxicity necessitates close monitoring of blood counts and may require dose adjustments or supportive measures such as growth factor support to mitigate the increased risk of infection and bleeding.

On the other hand, beta-blockers, proton pump inhibitors, and antihistamines do not share a similar risk profile when combined with doxorubicin. They generally do not have the same mechanism of action or overlapping toxicities that would lead to a compounded risk of significant side effects like myelosuppression. Therefore, while these medications may interact with doxorubicin in other ways, they do not pose an enhanced synergistic risk in the same manner as cyclophosphamide.