An 87-year-old male nursing home resident is currently undergoing antibiotic therapy for the treatment of a decubitus ulcer. One week into the treatment course, he experiences several episodes of watery diarrhea. Subsequent sigmoidoscopy demonstrates the presence of diffuse yellow plaques on the mucosa of the sigmoid colon. Which of the following is the best choice of treatment for this patient?

Oral trimethoprim/sulfamethoxazole

On the 4th day of hospital admission due to pneumonia, a 69-year-old woman develops non-bloody diarrhea and abdominal pain. She is currently treated with ceftriaxone. Despite the resolution of fever after the first 2 days of admission, her temperature is now 38.5°C (101.3°F). On physical examination, she has mild generalized abdominal tenderness without abdominal guarding or rebound tenderness. Laboratory studies show re-elevation of leukocyte counts. Ceftriaxone is discontinued. Given the most likely diagnosis in this patient, which of the following is the most sensitive test?

Nucleic acid amplification test

Stool culture for bacterial isolation and toxin presence

Enzyme immunoassay glutamate dehydrogenase

A 24-year-old man presents to the emergency department for severe abdominal pain for the past day. The patient states he has had profuse, watery diarrhea and abdominal pain that is keeping him up at night. The patient also claims that he sees blood on the toilet paper when he wipes and endorses having lost 5 pounds recently. The patient's past medical history is notable for IV drug abuse and a recent hospitalization for sepsis. His temperature is 99.5°F (37.5°C), blood pressure is 120/68 mmHg, pulse is 100/min, respirations are 14/min, and oxygen saturation is 98% on room air. On physical exam, you note a young man clutching his abdomen in pain. Abdominal exam demonstrates hyperactive bowel sounds and diffuse abdominal tenderness. Cardiopulmonary exam is within normal limits. Which of the following is the next best step in management?

Supportive therapy and ciprofloxacin if symptoms persist

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

A 72-year-old patient presents to the emergency department because of abdominal pain, diarrhea, and fever. He was started on levofloxacin for community-acquired pneumonia 2 weeks prior with resolution of his pulmonary symptoms. He has had hypertension for 20 years, for which he takes amlodipine. His temperature is 38.3°C (101.0°F), pulse is 90/min, and blood pressure is 110/70 mm Hg. On examination, mild abdominal distension with minimal tenderness was found. Laboratory tests reveal a peripheral white blood cell count of 12.000/mm3 and a stool guaiac mildly positive for occult blood. Which of the following best describe the mechanism of this patient illness?

Disruption of normal bowel flora and infection by spore-forming rods

Damage to the gastrointestinal tract by enteropathogenic viruses

Autoimmune inflammation of the rectum

Decreased blood flow to the gastrointestinal tract

Presence of osmotically active, poorly absorbed solutes in the bowel lumen

A 12-month-old boy is brought to the emergency department by his mother for several hours of crying and severe abdominal pain, followed by dark and bloody stools in the last hour. The mother reports that she did not note any vomiting or fevers leading up to this incident. She does report that the boy and his 7-year-old sister recently had “stomach bugs” but that both have been fine and that the sister has gone back to school. The boy was born by spontaneous vaginal delivery at 39 weeks and 5 days after a normal pregnancy. His temperature is 100.4°F (38.0°C), blood pressure is 96/72 mmHg, pulse is 90/min, respirations are 22/min. Which of the following was most likely to play a role in the pathogenesis of this patient’s disease?

Embolism to the mesenteric vessels

Failure of neural crest migration

Clostridium difficile infection for USMLE Step 3: High-Yield Microbiology Lessons

Pathophysiology - The Gut Wrecker

Causative Agent: Gram-positive, spore-forming, obligate anaerobic rod.
Transmission: Fecal-oral route. Spores are highly resilient, resisting heat, acid, and alcohol-based sanitizers.
Mechanism of Injury:
- Toxin A (Enterotoxin): Targets the brush border, causing inflammation, fluid secretion, and watery diarrhea.
- Toxin B (Cytotoxin): More potent; causes actin depolymerization, leading to mucosal cell apoptosis and necrosis.

C. difficile colitis: pseudomembrane and inflammation

⭐ The toxins' destruction of the colonic epithelium leads to the formation of characteristic pseudomembranes-yellow-white plaques composed of fibrin, mucus, and inflammatory cells.

Risk & Presentation - Antibiotic's Revenge

Risk Factors:
- Recent antibiotic use (📌 Clindamycin, Fluoroquinolones, Cephalosporins)
- Advanced age (>65 years)
- Recent hospitalization or long-term care residency
- Proton Pump Inhibitor (PPI) use
Classic Presentation:
- Profuse, watery, foul-smelling diarrhea (>3 loose stools in 24h)
- Lower abdominal pain & cramping
- Low-grade fever, leukocytosis
Severe Disease Hallmark:
- Pseudomembranous colitis (yellowish-white plaques on colonoscopy)

Endoscopic view of pseudomembranous colitis

⭐ While Clindamycin is classically cited, third-generation cephalosporins are a very common cause of hospital-acquired C. difficile.

Diagnosis - The Toxin Trackdown

Best Initial Test: Stool testing. Options include:
- Nucleic Acid Amplification Test (NAAT) for C. diff toxin genes.
- Glutamate Dehydrogenase (GDH) antigen + toxin EIA combo.
Severity Markers: Suggest severe disease.
- WBC > 15,000 cells/mL
- Creatinine ≥ 1.5x baseline
Imaging: Abdominal X-ray or CT for complications like toxic megacolon (colonic diameter > 6 cm).

⭐ NAAT is highly sensitive but doesn't distinguish between active infection and colonization. A positive toxin EIA confirms active toxin production, indicating true disease.

Management - The Flora Force

Primary Goal: Discontinue the inciting antibiotic as soon as possible.
Prevention: Strict contact precautions (gown, gloves).

⭐ Alcohol-based hand sanitizers do NOT kill C. difficile spores; vigorous hand washing with soap and water is required to physically remove them.

High‑Yield Points - ⚡ Biggest Takeaways

C. difficile is a Gram-positive, spore-forming anaerobe transmitted via spores, often following antibiotic use.

Key risk factors include clindamycin, fluoroquinolones, cephalosporins, and proton pump inhibitors.

Pathogenesis is mediated by Toxin A (enterotoxin) and Toxin B (cytotoxin), causing diarrhea and colonic damage.

Clinical presentation ranges from mild diarrhea to life-threatening pseudomembranous colitis and toxic megacolon.

Diagnosis relies on stool NAAT/PCR for toxin genes.

First-line treatment is oral fidaxomicin or oral vancomycin.

High recurrence rates are managed with fidaxomicin or a vancomycin taper.

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