A 65-year-old man undergoes cardiac bypass surgery and is placed on postoperative, broad-spectrum, antibiotic prophylaxis. Several days later, he develops fever, abdominal pain, and bloody diarrhea. Colonoscopic biopsy demonstrates a thick mucopurulent exudate. Which of the following is the most likely etiology of this patient’s gastrointestinal disorder?

**Core Concept**
The patient's symptoms and colonoscopic findings suggest an infectious gastrointestinal disorder that has occurred as a complication of antibiotic use. Antibiotic-associated diarrhea is a common side effect of broad-spectrum antibiotic prophylaxis, leading to an imbalance in the normal gut flora.

**Why the Correct Answer is Right**
The most likely etiology of this patient's gastrointestinal disorder is Clostridioides difficile (C. difficile) infection. C. difficile is a gram-positive, spore-forming bacterium that can cause a range of gastrointestinal symptoms, from mild diarrhea to life-threatening colitis. The use of broad-spectrum antibiotics disrupts the normal gut flora, allowing C. difficile to overgrow and produce toxins that damage the colonic mucosa.

C. difficile infection is characterized by the production of two main toxins, A and B, which cause the thick mucopurulent exudate seen on colonoscopy. The patient's symptoms of fever, abdominal pain, and bloody diarrhea are also consistent with C. difficile infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** While Escherichia coli (E. coli) can cause gastrointestinal infections, it is not typically associated with antibiotic-associated diarrhea or the specific colonoscopic findings seen in this patient.
* **Option B:** Salmonella species can cause gastrointestinal infections, but they are not typically associated with antibiotic-associated diarrhea or the specific colonoscopic findings seen in this patient.
* **Option D:** Shigella species can cause gastrointestinal infections, but they are not typically associated with antibiotic-associated diarrhea or the specific colonoscopic findings seen in this patient.

**Clinical Pearl / High-Yield Fact**
C. difficile infection is a common complication of antibiotic use, particularly in hospitalized patients. It is essential to consider C. difficile infection in patients who develop diarrhea or colitis after antibiotic use, and to test for the toxin using a stool test or rectal swab.

**Correct Answer:** C. Clostridioides difficile.

✓ Correct Answer: B. Clostridium difficile