The correct answer is:
A) Amebic colitis
This patient has amebic colitis caused by Entamoeba histolytica. The diagnosis is suggested by travel to an endemic area, bloody diarrhea, abdominal pain, low-grade fever, weight loss, and negative routine bacterial stool cultures. Unlike giardiasis, which causes a noninflammatory diarrheal illness characterized by bloating and malabsorption, amebiasis is an invasive infection that produces colonic inflammation and dysentery.
After ingestion of mature cysts, E. histolytica trophozoites invade the colonic mucosa, causing tissue destruction and ulceration. Patients typically present with gradual-onset abdominal pain, bloody diarrhea, and constitutional symptoms. Colonoscopy, if performed, may demonstrate characteristic flask-shaped ulcers, a classic board finding. The organism can also spread hematogenously through the portal circulation, producing liver abscesses, which represent the most common extraintestinal manifestation of amebiasis.
Diagnosis is often established with stool antigen testing, molecular testing, or serology. Treatment requires eradication of both tissue-invasive trophozoites and intraluminal cysts. Metronidazole is used for invasive disease and must be followed by a luminal agent such as paromomycin to eliminate residual cysts and prevent recurrence.
A high-yield Step 2 distinction is that inflammatory diarrhea with blood and mucus suggests mucosal invasion, whereas Giardia causes nonbloody diarrhea with bloating and malabsorption.
Answer choice B: Giardiasis, is incorrect.
Giardiasis is a common distractor because both infections may follow exposure to contaminated food or water. However, Giardia typically causes nonbloody diarrhea, bloating, flatulence, and steatorrhea rather than dysentery with blood and mucus.
Answer choice C: Irritable bowel syndrome, is incorrect.
IBS can cause chronic abdominal discomfort and altered bowel habits, but it does not cause fever, weight loss, anemia, or bloody diarrhea. These findings are considered alarm features and suggest organic disease.
Answer choice D: Microscopic colitis, is incorrect.
Microscopic colitis generally presents with chronic watery, nonbloody diarrhea in older adults. It is not associated with invasive colonic infection, fever, or hematochezia.
Answer choice E: Ulcerative colitis, is incorrect.
Ulcerative colitis is a reasonable consideration because it can present with bloody diarrhea and abdominal pain. However, the recent travel history, acute presentation, and infectious risk factors make amebic colitis more likely. Additionally, infectious causes should be excluded before diagnosing inflammatory bowel disease.
Key Learning Point
Amebiasis caused by Entamoeba histolytica produces invasive colitis characterized by abdominal pain, bloody diarrhea, and dysentery. A classic complication is a hepatic abscess, and treatment requires metronidazole followed by a luminal amebicidal agent.
