Which oncologic emergency should the nurse suspect in a patient presenting with a high fever, tachycardia, and leukocytosis after chemotherapy?

The scenario presented suggests that the patient is experiencing symptoms consistent with an infection, particularly following chemotherapy, which can severely compromise the immune system. A high fever, tachycardia, and leukocytosis indicate a systemic response to an infection, often referred to as an inflammatory response. Septic shock can develop when an infection leads to systemic inflammation, resulting in the body's organs receiving insufficient blood flow, potentially leading to multi-organ failure.

Chemotherapy can decrease white blood cell counts (particularly neutrophils), increasing vulnerability to infections. When a patient presents with leukocytosis in this context, it further supports the presence of an infection rather than malignancy-related complications such as tumor lysis syndrome or malignant bowel obstruction. Cardiac tamponade, while a serious condition that can occur in cancer patients, does not typically present with the fever and leukocytosis described.

Therefore, given the clinical signs and the recent chemotherapy, the most likely oncologic emergency within this situation is septic shock, as it aligns with the manifestations of systemic infection and the complications related to immunosuppression.