Immune-related adverse event (irAE): Nivolumab is an immune checkpoint inhibitor that can cause a variety of immune-related adverse events, including pneumonitis, colitis, hepatitis, and endocrinopathies. The onset of symptoms one week after starting nivolumab is consistent with an irAE, which can occur at any time during treatment but often presents within the first few weeks.

Progression of underlying disease: The patient's deterioration could be due to progression of the underlying cancer, which may not be responding to nivolumab.
Infection: Patients on immunotherapy may be at increased risk of infections, which could present with non-specific symptoms such as fever, fatigue, and deterioration.
Medication side effects: Nivolumab can cause a range of side effects, including fatigue, rash, and diarrhea, which could contribute to the patient's deterioration.

Pulmonary embolism: Although less likely, a pulmonary embolism could present with sudden deterioration and would be a life-threatening condition that requires immediate attention.
Sepsis: Sepsis is a potentially life-threatening condition that could present with non-specific symptoms and would require prompt recognition and treatment.
Adrenal crisis: Immune checkpoint inhibitors like nivolumab can cause adrenal insufficiency, which could present with non-specific symptoms such as fatigue, weakness, and deterioration.

Hemophagocytic lymphohistiocytosis (HLH): HLH is a rare but life-threatening condition that can be triggered by immune checkpoint inhibitors.
Cytokine release syndrome: This is a rare but potentially life-threatening condition that can occur with immune checkpoint inhibitors, characterized by a systemic inflammatory response.
Neurological irAEs: Although rare, nivolumab can cause neurological irAEs such as meningitis, encephalitis, or myasthenia gravis, which could present with non-specific symptoms such as confusion, weakness, or deterioration.