A 72-year-old man presents to an outpatient clinic with a 5-day history of progressively worsening frequent, nonbloody, watery stools, now occurring 8-10 times daily. He reports general malaise and decreased oral intake. Two weeks prior, he was hospitalized for community-acquired pneumonia and completed a 10-day course of intravenous clindamycin. His medical history includes type 2 diabetes mellitus managed with metformin and essential hypertension managed with lisinopril. On physical examination, his temperature is 37.8°C (100.0°F), blood pressure is 105/70 mmHg, pulse is 102/min, respirations are 16/min, and oxygen saturation is 97% on room air. He appears mildly fatigued with dry mucous membranes. Abdominal examination reveals diffuse mild tenderness to palpation without rebound, rigidity, or guarding. Bowel sounds are hyperactive. Given this clinical presentation, what is the most appropriate initial diagnostic approach and subsequent management strategy?

A 68-year-old woman is admitted to the internal medicine ward with a 4-day history of frequent, watery bowel movements, accompanied by intermittent abdominal cramping and low-grade fever. She reports no recent foreign travel or consumption of unusual foods, and her household contacts remain asymptomatic. Her medical history includes well-controlled hypertension and type 2 diabetes. She was recently discharged from the hospital three weeks prior following a bout of pneumonia, for which she received a 7-day course of intravenous ceftriaxone. She also takes daily omeprazole for chronic reflux. On physical examination, she appears fatigued, with dry mucous membranes and mild diffuse abdominal tenderness without rebound or guarding. Bowel sounds are hyperactive. Her blood pressure is 100/60 mmHg, heart rate 98 bpm, respiratory rate 18 breaths/min, and temperature 37.9°C. Given her presentation and history, which diagnostic approach and initial therapeutic intervention would be most appropriate for this patient, and what potential complications should be immediately monitored for?

A 68-year-old woman presents to an outpatient internal medicine clinic complaining of significant changes in her bowel habits over the past five days. She describes experiencing frequent, watery, and malodorous stools, occurring 8-10 times daily, accompanied by increasing abdominal cramping. Her medical history is notable for a recent hospitalization three weeks ago for community-acquired pneumonia, for which she received a 7-day course of intravenous ceftriaxone followed by oral azithromycin. She denies any recent travel, consumption of unpurified water, or undercooked food. On examination, her temperature is 38.3°C, blood pressure is 105/65 mmHg, pulse is 108/min, respirations are 18/min, and oxygen saturation is 97% on room air. Abdominal examination reveals mild diffuse tenderness, especially in the lower quadrants, with hyperactive bowel sounds. Given her presentation, what is the most likely diagnosis and the appropriate initial management strategy?

A 72-year-old woman, currently admitted to a medical ward following recent treatment for pyelonephritis with intravenous broad-spectrum antibiotics, now presents with a worsening clinical picture. Over the past 5 days, she has developed profuse, watery diarrhea, occurring 10-12 times daily, accompanied by diffuse, severe abdominal pain that she rates as 9/10 in intensity. She denies any prior history of similar gastrointestinal issues. Her medical history includes well-controlled hypertension managed with amlodipine. She has no significant surgical history. Her vital signs are notable for a temperature of 39.8°C, blood pressure of 95/55 mmHg, pulse of 118 beats/min, and respirations of 20 breaths/min. On physical examination, her abdomen is distended and diffusely tender to palpation, particularly in the lower quadrants, with definite guarding and rebound tenderness. Bowel sounds are diminished. Considering these findings, what is the most likely diagnosis and the immediate, critical therapeutic intervention?

A 68-year-old woman presents to her primary care physician's office with severe diarrhea. She reports experiencing approximately 12 episodes of non-bloody, watery stools daily for the past 4 days. Her medical history includes a recent hospitalization for community-acquired pneumonia, for which she completed a 10-day course of intravenous antibiotics 5 days prior to symptom onset. On examination, her temperature is 37.8°C, blood pressure is 95/50 mmHg, pulse is 112/min, respirations are 18/min, and oxygen saturation is 97% on room air. Abdominal examination reveals diffuse tenderness to palpation, particularly in the lower quadrants, but no guarding or rebound. Capillary refill is sluggish at 3 seconds. Rectal examination is unremarkable. Given these findings, what is the most likely diagnosis and the most appropriate initial pharmacological management strategy for this patient?