A 65-year-old patient with a history of chronic constipation is admitted to the hospital. The patient reports a sudden change in bowel habits, including diarrhea and abdominal pain. What is the MOST appropriate initial nursing intervention?

Obtaining a stool sample for analysis is the most appropriate initial nursing intervention because a sudden change in bowel habits — from chronic constipation to diarrhea with abdominal pain — in a 65-year-old patient warrants immediate investigation for potentially serious causes including Clostridioides difficile infection, colorectal cancer, or infectious gastroenteritis. Assessment and data collection must precede therapeutic intervention; stool analysis provides the diagnostic information needed to guide treatment safely. Encouraging ambulation is a reasonable non-pharmacological strategy for promoting bowel motility in constipation but does not address the acute change or its underlying cause. Increasing fiber intake is similarly appropriate for chronic constipation management but is premature and potentially harmful if an infectious or obstructive process is present, as fiber can worsen certain GI conditions. Administering an anti-diarrheal medication should not be done before knowing the cause of the diarrhea; if an infection such as C. difficile is present, suppressing diarrhea can delay pathogen clearance, worsen toxin retention, and increase the risk of toxic megacolon. The nursing process requires assessment before intervention, and obtaining a stool specimen is the first step in accurate assessment of this patient's acute change in condition.