A 61-year-old white man presents to the emergency department with a 1-week history of diffuse "crampy" abdominal pain. For the past 2 months, he has had diarrhea and has been passing blood with every bowel movement, either on the stool surface or separately from the stool. Recently, he has had 10-15 loose stools per day, with associated urgency and tenesmus. He has lost 4 lb in the past week. He feels lightheaded upon standing and is experiencing fatigue and mild shortness of breath. He takes atorvastatin for hypercholesteremia and acetaminophen for intermittent back and knee pain. He is allergic to penicillin. He has no history of gastrointestinal tract disease; screening colonoscopy performed 2 years ago was unremarkable. His father died of myocardial infarction when he was in his 60s, and his mother died of breast cancer when she was in her 40s. He has no siblings. He is retired from his work as an accountant and lives with his wife. He has not travelled recently. He has a 25-pack-year smoking history but has not smoked for approximately 10 years. He drinks alcohol on social occasions and on weekends, and he denies illicit drug abuse.

On physical examination, the patient is a pale, diaphoretic man in some distress due to abdominal pain. His oral temperature is 101.5°F (38.6°C), blood pressure is 110/62 mm Hg, pulse is 107 beats/min, and respiratory rate is 24 breaths/min. His body mass index is 29.7 kg/m2. Examination of his head and neck is unremarkable aside from pale mucosal membranes. Lung auscultation reveals normal breath sounds, with no crackles or rhonchi. His heart rate is tachycardic, and his heart rhythm is regular. There are no murmurs or extra heart sounds. His abdomen is distended and firm, with diffuse rebound tenderness that seems worse in the lower left quadrant. Bowel sounds are diminished. No hepatosplenomegaly, ascites, or palpable masses are appreciated. Digital rectal examination demonstrates normal rectal tone, "velvety" rectal mucosa, and bright red blood on withdrawal of the examining finger. There is pitting edema of the legs and feet.

Laboratory analysis includes a complete blood count (CBC), which demonstrates anemia (hemoglobin concentration of 6.7 g/dL [67 g/L]), leukocytosis (leukocyte count of 14.2 x 103 cells/μL [14.2 x 109 cells/L]), and thrombocytosis (platelet count of 540 x 103 cells/µL [540 x 109 cells/L]). Additional findings include elevations in the erythrocyte sedimentation rate (78 mm/h) and C-reactive protein level (114 mg/L). His albumin level is low, at 2.0 g/dL (20 g/L). The basic metabolic panel is unremarkable. Repeat stool cultures and samples for ova and parasites are negative. Flexible sigmoidoscopy shows diffusely erythematous and friable colonic mucosa with large ulcerated patches, abundant yellow-white exudate, and oozing blood. These findings extend in a proximal and continuous fashion from the rectum to at least the middle of the sigmoid colon. Tissue biopsy samples sent for histopathologic examination reveal lymphoplasmacytic inflammation along the base of the crypts; neutrophils that infiltrate the crypt epithelium and collect in the depths of the colonic crypts to form "crypt abscesses"; and focal chronic reactive changes, including crypt branching, gland atrophy, and goblet-cell mucin depletion. No granulomas are identified.

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What is the most likely diagnosis?

Hint: Note the frequent passage of bloody loose stools for more than 2 months in the setting of fever, tachycardia, and anemia.
Colorectal cancer
Microscopic colitis
Inflammatory bowel disease
Hemorrhoids
Clostridium difficile colitis

Inflammatory bowel disease

А почему не Microscopic colitis ?

Пожалуй так и есть.
Микроскопический (лимфоцитарный) колит.