A 75-year-old man presents with mild cramping pain in the left lower abdomen for 1 day. During the same time, he passed stools twice, well mixed with fresh blood. His surgical history is significant for removal of a single adenomatous polyp during a screening colonoscopy 2 years ago. In addition, he was diagnosed with hypertension 5 years ago, and stage III chronic kidney disease 3 years ago, for which he is being followed up regularly. His smoking history amounts to 7.5 pack years. He only drinks socially and denies recreational drug abuse. Initial laboratory studies are as follows:

WBC: 12,000/mm³ (4500-10,000) Segmented neutrophils: 70% (54-62)

Hemoglobin: 12.5 g/dL (13.5-17.5)

Hematocrit: 38% (41-53)

Platelets: 200,000/mm³ (150,000-400,000)

PT: 14 seconds (11-15)

aPTT: 30 seconds (25-40)

Na⁺: 136 mEq/L (136-146)

K⁺: 5.1 mEq/L (3.5-5.0)

Cl^-: 103mEq/L (95-105)

HCO₃^-: 24 mEq/L (22-28)

Glucose: 100mg/dL

BUN: 25 mg/dL (7-18)

Creatinine: 2.0 mg/dL (0.6-1.2)

Calcium: 9.8 mg/dL (8.4-10.2)

Protein: 7.0 g/dL (6.0-7.8)

Albumin: 4.5 g/dL (3.5-5.5)

ALT: 36 U/L (10-40)

AST: 30 U/L (12-38)

Alkaline phosphatase: 90 U/L (25-100)

Bilirubin: 0.9 mg/dL (0.1-1.0)

Amylase: 94 U/L (25-125)