A 32-year-old female comes to the physician because of recurrent episodes of abdominal pain, bloating, and loose stools lasting several days to a couple weeks. She has had these episodes since she was 24 years old but they have worsened over the last 6 weeks. The site of the abdominal pain and the intensity of pain vary. She has around 3–4 bowel movements per day during these episodes. Menses are regular at 31 day intervals with moderate flow; she has moderate pain in her lower abdomen during menstruation. She moved from a different city 2 months ago to start a new demanding job. Her mother has been suffering from depression for 10 years. She does not smoke or drink alcohol. Her own medications include multivitamins and occasionally naproxen for pain. Temperature is 37.4°C (99.3°F), pulse is 88/min, and blood pressure is 110/82 mm Hg. Abdominal examination shows no abnormalities. Laboratory studies show: Hemoglobin 14.1 g/dL Leukocyte count 8100/mm3 Erythrocyte sedimentation rate 15 mm/h Serum Glucose 96 mg/dL Creatinine 1.1 mg/dL IgA anti-tissue transglutaminase antibody negative Urinalysis shows no abnormalities. Further evaluation is most likely to show which of the following in this patient?

Relief of abdominal pain after defecation

A 24-year-old man comes to the physician with a 2-day history of fever, crampy abdominal pain, and blood-tinged diarrhea. He recently returned from a trip to Mexico. His temperature is 38.2°C (100.8°F). Abdominal examination shows diffuse tenderness to palpation; bowel sounds are hyperactive. Stool cultures grow nonlactose fermenting, oxidase-negative, gram-negative rods that do not produce hydrogen sulfide on triple sugar iron agar. Which of the following processes is most likely involved in the pathogenesis of this patient's condition?

Inhibition of host cytoskeleton organization

Overactivation of adenylate cyclase

Flagella-mediated gut colonization

Invasion of colonic microfold cells

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 52-year-old woman presents to the clinic with several days of vomiting and diarrhea. She also complains of headaches, muscle aches, and fever, but denies tenesmus, urgency, and bloody diarrhea. Past medical history is insignificant. When asked about any recent travel she says that she just came back from a cruise ship vacation. Her temperature is 37°C (98.6° F), respiratory rate is 15/min, pulse is 67/min, and blood pressure is 122/98 mm Hg. Physical examination is non-contributory. Microscopic examination of the stool is negative for ova or parasites. What is the most likely diagnosis?

Traveler’s diarrhea due to ETEC

Irritable bowel syndrome for USMLE Step 3: High-Yield Internal Medicine Lessons

IBS Overview - The Gut Feeling

Chronic, relapsing functional GI disorder characterized by abdominal pain and altered bowel habits without demonstrable organic pathology.
Pathophysiology: Multifactorial, involving visceral hypersensitivity, altered gut motility, gut-brain axis dysregulation, and post-infectious changes.
Diagnosis: Relies on clinical symptoms using Rome IV criteria:
- Recurrent abdominal pain on average at least 1 day/week in the last 3 months.
- Associated with ≥2 of the following: related to defecation, change in stool frequency, or change in stool form (appearance).

⭐ IBS is one of the most common GI disorders, predominantly affecting women and individuals < 50 years old. It is a diagnosis of exclusion.

Gut-Brain Axis Dysregulation in IBS

Pathophysiology - Gut-Brain Axis Glitch

Core Defect: Bidirectional signaling failure between the central nervous system (CNS) and the enteric nervous system (ENS).
Key Consequences:
- Visceral Hypersensitivity: Amplified pain signals from normal gut events (e.g., gas, peristalsis). The primary reason for abdominal pain.
- Altered Motility: Irregular gut contractions leading to diarrhea, constipation, or a mix.
- Immune Activation: Low-grade inflammation and mast cell activation.
- Microbiome Dysbiosis: Alterations in gut flora composition.
- Neurotransmitter Dysregulation: Primarily involving Serotonin (5-HT).

Gut-Brain Axis in IBS: Pathophysiology and Risk Factors

⭐ Post-infectious IBS (PI-IBS) can occur after acute gastroenteritis (e.g., Campylobacter), suggesting an autoimmune link where antibodies to microbial toxins cross-react with gut proteins like vinculin.

Diagnosis - The Rome Rules

Diagnosis of IBS is based on the Rome IV criteria, focusing on recurrent abdominal pain.

Frequency: On average, at least 1 day/week in the last 3 months.
Association: Pain must be associated with ≥2 of the following:
- Related to defecation
- Change in stool frequency
- Change in stool form (appearance)
Chronicity: Criteria fulfilled for the last 3 months with symptom onset at least 6 months prior to diagnosis.

⭐ Absence of "alarm features" (e.g., rectal bleeding, nocturnal diarrhea, weight loss, anemia) is key to diagnosis. Their presence suggests organic disease requiring further investigation.

Management - Taming the Tumult

Foundation: Patient education, stress reduction, and increased physical activity.
Dietary: A trial of a low FODMAP diet is a cornerstone therapy. Avoid known trigger foods and excessive caffeine.
Symptom-Guided Pharmacotherapy:
- IBS-C: Soluble fiber (psyllium), osmotic laxatives (PEG). For refractory cases, consider secretagogues like Lubiprostone.
- IBS-D: Loperamide for acute control. Rifaximin, a gut-specific antibiotic, can improve bloating and diarrhea.
- Pain/Bloating: Antispasmodics (e.g., dicyclomine) provide short-term relief.

⭐ For refractory IBS with severe pain, use central neuromodulators. Low-dose TCAs (amitriptyline) are ideal for IBS-D due to their constipating side effects.

IBS is a functional disorder defined by chronic, relapsing abdominal pain related to defecation and altered bowel habits.

Diagnosis relies on the Rome IV criteria after excluding organic causes; look for the absence of red flag symptoms like weight loss or bleeding.

Key pathophysiology involves visceral hypersensitivity and altered gut-brain axis communication.

Management is tailored to the predominant symptom: fiber for constipation (IBS-C) and loperamide for diarrhea (IBS-D).

Psychosocial stressors frequently exacerbate symptoms.

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Irritable bowel syndrome