Serotonin Syndrome: This condition is a likely diagnosis given the patient's use of Lexapro (escitalopram), an SSRI, which can increase serotonin levels. The combination of altered mental status and muscle rigidity, along with the potential for other symptoms such as fever, autonomic instability, and changes in blood pressure, makes serotonin syndrome a strong consideration. The patient's age and polypharmacy also increase the risk.

Seizure or Post-Ictal State: Given the patient's history of seizure disorder, a recent seizure could explain the altered mental status. Muscle rigidity could be part of the seizure activity or a post-ictal phenomenon.
Eliquis (Apixaban) Related Bleed: Although less common, the use of anticoagulants like Eliquis increases the risk of intracranial hemorrhage, which could present with altered mental status and potentially with muscle rigidity if there is increased intracranial pressure or direct brainstem involvement.
Metabolic Encephalopathy: Renal cell carcinoma and its treatment, along with the patient's age and potential renal impairment, increase the risk of metabolic disturbances (e.g., hypercalcemia, hyponatremia) that could lead to encephalopathy.

Intracranial Hemorrhage: This is a critical diagnosis to consider due to the patient's anticoagulation therapy. Even though it might not be the first consideration based on the symptoms alone, the potential consequences of missing this diagnosis are severe.
Status Epilepticus: Continuous or recurrent seizure activity without full recovery between seizures is a medical emergency. The patient's history of seizure disorder and the presentation of altered mental status and muscle rigidity necessitate considering this potentially life-threatening condition.
Meningitis or Encephalitis: Infections of the central nervous system can present with altered mental status and, less commonly, with muscle rigidity. Given the severity of these conditions, they must be considered, especially in an elderly patient with potential immune compromise due to cancer.

Neuroleptic Malignant Syndrome (NMS): Although the patient is not on typical neuroleptics, some anticonvulsants and other medications can rarely induce NMS, characterized by fever, muscle rigidity, altered mental status, and autonomic dysfunction.
Lamictal or Keppra Toxicity: While less common, toxicity from these medications, especially if levels become elevated due to renal impairment or drug interactions, could potentially cause altered mental status and other neurological symptoms.
Paraneoplastic Syndromes: Rarely, renal cell carcinoma can be associated with paraneoplastic syndromes that affect the nervous system, such as encephalitis or stiff person syndrome, which could explain the patient's symptoms.