Source identification and control

A 35-year-old woman with a medical history significant for asthma, hypertension, and occasional IV drug use comes to the emergency department with fever. On physical exam, there are findings depicted in figure A, for which the patient cannot account. What test will be most helpful to establish the diagnosis?

A 72-year-old female presents to the emergency department complaining of severe abdominal pain and several days of bloody diarrhea. Her symptoms began with intermittent bloody diarrhea five days ago and have worsened steadily. For the last 24 hours, she has complained of fevers, chills, and abdominal pain. She has a history of ulcerative colitis, idiopathic hypertension, and hypothyroidism. Her medications include hydrochlorothiazide, levothyroxine, and sulfasalazine. In the ED, her temperature is 39.1°C (102.4°F), pulse is 120/min, blood pressure is 90/60 mmHg, and respirations are 20/min. On exam, the patient is alert and oriented to person and place, but does not know the day. Her mucus membranes are dry. Heart and lung exam are not revealing. Her abdomen is distended with marked rebound tenderness. Bowel sounds are hyperactive. Serum: Na+: 142 mEq/L Cl-: 107 mEq/L K+: 3.3 mEq/L HCO3-: 20 mEq/L BUN: 15 mg/dL Glucose: 92 mg/dL Creatinine: 1.2 mg/dL Calcium: 10.1 mg/dL Hemoglobin: 11.2 g/dL Hematocrit: 30% Leukocyte count: 14,600/mm^3 with normal differential Platelet count: 405,000/mm^3 What is the next best step in management?