Review Question - QID 213876

A 55-year-old woman is admitted into the hospital for progression of breast cancer. During her hospital stay, she is found to be more lethargic, appears depressed, and is less interactive with her family. There were no recent medication changes, and she has not had fevers during her hospital stay. Her medical history is significant for stage IV breast cancer complicated by metastasis to her brain and bones. She also has a history of hypertension and hyperlipidemia. On neurologic examination, the patient is awake but is not interactive. She becomes somnolent during the examination. She is oriented to self but not place or date. She has poor attention and concentration and is able to follow simple commands. She moves all four of her extremities against gravity but shows poor effort to engage in motor confrontational testing. She withdraws to noxious stimuli in all her extremities. When her arms are outstretched, she is noted to have asterixis. An electroencephalogram is performed which demonstrates diffuse cerebral slowing and triphasic waves. Laboratory studies are shown below:

Hemoglobin: 14 g/dL
Hematocrit: 41%
Leukocyte count: 8,500/mm^3 with normal differential
Platelet count: 220,000/mm^3

Serum:
Na+: 141 mEq/L
Cl-: 98 mEq/L
K+: 3.8 mEq/L
HCO3-: 24 mEq/L
BUN: 29 mg/dL
Glucose: 140 mg/dL
Creatinine: 1.1 mg/dL

Which of the following is the most likely cause of her poor mental status?