Review Question - QID 210522

A 24-year-old man presents to the emergency department for bloody diarrhea for the past 3 days without improvement. He recently returned from a camping trip where he drank stream water and admits to eating undercooked meats which included beef, chicken, pork, and salmon. The patient's father died at age 40 due to colon cancer, and his mother died of breast cancer at the age of 52. The patient lives alone and drinks socially. His temperature is 98.3°F (36.8°C), blood pressure is 107/58 mmHg, pulse is 127/min, respirations are 12/min, and oxygen saturation is 99% on room air. Laboratory values are ordered as seen below.

Hemoglobin: 9.2 g/dL
Hematocrit: 29%
Leukocyte count: 9,500/mm^3 with normal differential
Platelet count: 87,000/mm^3
Lactate dehydrogenase: 327 IU/L
Haptoglobin: 5 mg/dL

Serum:
Na+: 139 mEq/L
Cl-: 100 mEq/L
K+: 5.9 mEq/L
HCO3-: 19 mEq/L
BUN: 59 mg/dL
Glucose: 99 mg/dL
Creatinine: 1.1 mg/dL
Ca2+: 10.2 mg/dL

Which of the following is the most likely cause of this patient's presentation?