A 29-year-old man presents to his primary care physician with 3 weeks of diarrhea, abdominal bloating, and excessive flatulence. He recently returned from a backpacking trip in the Rocky Mountains, where he frequently drank water from streams and lakes after only minimal filtration. He denies fever, hematochezia, or significant abdominal pain. His medical history is unremarkable, and he takes no medications. Temperature is 36.8°C (98.2°F), blood pressure is 122/74 mm Hg, pulse is 76/min, and respiratory rate is 14/min. Physical examination reveals mild diffuse abdominal distention without tenderness. Laboratory studies are unremarkable. Stool examination is negative for leukocytes and occult blood.
Which of the following is the most likely diagnosis?
The correct answer is:
C) Giardiasis
This patient has classic giardiasis caused by Giardia lamblia. The key clues are exposure to untreated freshwater during a camping trip, prolonged diarrhea, bloating, flatulence, and mild weight loss. Unlike invasive bacterial enteric infections, giardiasis typically causes noninflammatory diarrhea, so fever, bloody stools, fecal leukocytes, and severe abdominal pain are usually absent.
Giardia is a flagellated protozoan that colonizes the proximal small intestine. The organism adheres to the intestinal mucosa and disrupts normal absorptive function, resulting in malabsorption. Patients often develop foul-smelling diarrhea, bloating, excessive gas production, and steatorrhea. Because fat absorption is impaired, prolonged infection may lead to weight loss and nutritional deficiencies.
A high-yield board association is exposure to contaminated water from lakes, rivers, streams, or daycare settings. Diagnosis can be established with stool antigen testing, nucleic acid amplification testing, or identification of cysts and trophozoites in stool specimens. First-line treatment is typically metronidazole, tinidazole, or nitazoxanide.
Step 2 frequently tests the distinction between inflammatory and noninflammatory diarrheal illnesses. Giardiasis causes a malabsorptive syndrome with bloating and flatulence rather than fever and bloody diarrhea.
Answer choice A: Amebic colitis, is incorrect.
Amebiasis caused by Entamoeba histolytica typically produces invasive colitis with abdominal pain and bloody diarrhea. Patients often have a history of travel to endemic regions or exposure to contaminated food and water. The absence of bloody stools and the prominent bloating favor giardiasis.
Answer choice B: Clostridioides difficile infection, is incorrect.
C. difficile infection usually occurs after recent antibiotic exposure or hospitalization and often presents with watery diarrhea, abdominal pain, fever, and leukocytosis. This patient’s wilderness water exposure is much more characteristic of giardiasis.
Answer choice D: Salmonella gastroenteritis, is incorrect.
Salmonella commonly causes an acute inflammatory diarrhea associated with fever, nausea, abdominal cramping, and sometimes bloody stools. Symptoms are typically more acute and less associated with chronic bloating and malabsorption.
Answer choice E: Ulcerative colitis, is incorrect.
Ulcerative colitis generally causes chronic inflammatory diarrhea with hematochezia, urgency, and mucosal inflammation on colonoscopy. The environmental exposure history and malabsorptive symptoms point toward an infectious etiology instead.
Key Learning Point
Giardiasis is a noninvasive protozoal infection acquired through contaminated water and causes prolonged diarrhea, bloating, flatulence, and malabsorption. Exposure to untreated freshwater is a classic board clue.
