A man working in a constructing company presented with watery, foul-smelling diarrhea since 3 weeks. There is no blood in the stools. If it is giardiasis, it is best diagnosed by:

Giardia lamblia causes giardiasis. Watery (nonbloody), foul-smelling diarrhea is accompanied by nausea, anorexia, flatulence, and abdominal cramps persisting for weeks or months. There is no fever. Diagnosis is made by finding trophozoites or cysts or both in diarrheal stools. In formed stools (e.g., in asymptomatic carriers), only cysts are seen. An ELISA test that detects a Giardia cyst wall antigen in the stool is also very useful. Tests for antibody in the serum are not routinely available. Ref: Levinson W. (2012). Chapter 51. Intestinal & Urogenital Protozoa. In W. Levinson (Ed), Review of Medical Microbiology & Immunology, 12e.