Which of the following statements about Giardia duodenalisiscorrect?

• A. Giardia bacteria can survive for months outside the body in their trophozoite stage.
• B. Giardia represents the most common helminthic infection in the United States.
• C. Giardia cysts hatch into active trophozoites as they move into the human large intestine.
• D. Depending on the route of infection, giardiasis may be classified as either a zoonotic or a non-zoonotic disease. 

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the organism Giardia duodenalis. It is a protozoan parasite, not a bacterium or a helminth, so statements referring to it as bacteria or helminthic infection are incorrect.
2. Step 2: Recall the life cycle of Giardia. It exists in two main forms: the cyst and the trophozoite. The cyst is the infectious, environmentally resistant form, while the trophozoite is the active, motile form inside the host.
3. Step 3: Note that Giardia cysts survive outside the host for extended periods, not the trophozoites. Trophozoites are fragile and do not survive long outside the body.
4. Step 4: Recognize that Giardia cysts excyst (not hatch) in the small intestine, releasing trophozoites. The large intestine is not the primary site for excystation or trophozoite activity.
5. Step 5: Understand the epidemiology of giardiasis. It can be transmitted via zoonotic routes (from animals to humans) or non-zoonotic routes (human to human), depending on the source of infection, making the statement about classification based on infection route correct.