An 82-year-old woman presents to the emergency department because of large-volume diarrhea for the past three days. The patient has had at least five watery bowel movements per day. She has not had any blood in the stool. Medical history is notable for hypertension, osteoarthritis, and a recent episode of aspiration pneumonia, for which she was prescribed clindamycin. Vitals are unremarkable. Mucous membranes appear dry. No focal tenderness is elicited with palpation of the abdomen. Which of the following is the most likely cause of this patient’s symptoms?

Enteroinvasive Escherichia coli

Giardia lamblia

Foundational SciencesMicrobiologyParasitologyProtozoaGastrointestinal infections

Organ SystemsGastrointestinal systemMicrobiologyParasitic

USMLE® Step 1 ReviewGastrointestinal systemMicrobiologyParasitic

USMLE® Step 1 ReviewHuman developmentMicrobial biology and pharmacologyParasitologyProtozoa

USMLE® Step 2 ReviewGastrointestinal systemBack to the Basic SciencesMicrobiologyParasitic

Definitions & Key takeaways

Giardia lamblia is a protozoan parasite that causes the diarrheal disease giardiasis. It classically affects campers and hikers after contaminated water, causing symptoms like bloating, foul-smelling diarrhea, and flatulence. It is treated with antibiotics, and its prevention includes good hygiene practices and prompt treatment of water supplies that may be contaminated with Giardia.

Giardia lamblia

Definitions & Key takeaways

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