Clostridioides difficile infection (CDI): Given the patient's history of chronic nonbloody diarrhea that did not improve with TMP-SMX, and the presence of leukocytosis with a left shift, hypothermia, and elevated procalcitonin, CDI is a strong consideration. The use of antibiotics like TMP-SMX is a known risk factor for developing CDI.

Inflammatory bowel disease (IBD): Although the patient's age and lack of bloody diarrhea might be less typical, IBD (including Crohn's disease and ulcerative colitis) can present at any age and with varying symptoms. The chronic nature of the diarrhea and lack of response to antibiotics could suggest an inflammatory cause.
Malabsorption or small intestine bacterial overgrowth (SIBO): These conditions can cause chronic diarrhea and might not respond to standard antibiotic treatments. The patient's age and the chronic nature of symptoms could align with these diagnoses.
Giardiasis or other parasitic infections not detected by initial parasitologic exam: Although the initial parasitologic exam was negative, some parasites might require repeated testing or special staining techniques for detection. Giardiasis, for example, can cause chronic diarrhea and might not always be detected on the first test.

Neutropenic colitis (typhlitis): This condition is a medical emergency and can occur in patients with neutropenia, which might not be immediately apparent. The presence of leukocytosis with a left shift and elevated procalcitonin could indicate a severe infection.
Ischemic colitis: This condition can present with diarrhea and abdominal pain, and while less common, it is critical to diagnose promptly due to the risk of bowel ischemia and necrosis.
Colorectal cancer: Although less likely to present with diarrhea as the sole symptom, colorectal cancer can cause chronic changes in bowel habits. Given the patient's age, it is essential to consider this diagnosis, especially if other symptoms or signs suggestive of cancer are present.

Whipple's disease: A rare, systemic bacterial infection that can cause chronic diarrhea, weight loss, and malabsorption. It is essential to consider in cases of chronic diarrhea with no clear cause after extensive evaluation.
Lymphocytic or collagenous colitis: These are forms of microscopic colitis that can cause chronic, non-bloody diarrhea. They are more common in older adults but are less likely than other causes listed here.
Chronic intestinal pseudo-obstruction: A rare condition characterized by recurrent episodes of intestinal obstruction without any physical blockage. It can cause chronic diarrhea among other symptoms.