Which parasite is associated with a "coffee ground" appearance in vomit?

The parasite associated with a "coffee ground" appearance in vomit is Entamoeba histolytica. This characteristic appearance is indicative of bleeding in the gastrointestinal tract, which can occur due to the ulcerative lesions caused by this intestinal protozoan. Entamoeba histolytica is known to invade the intestinal lining, leading to dysentery, where the presence of blood in the vomit reflects the extent of tissue damage.

In the context of the other organisms listed, Giardia lamblia primarily causes watery diarrhea and does not typically result in gastrointestinal bleeding. Toxoplasma gondii is mostly associated with flu-like symptoms and can affect various organs but does not create this specific vomiting symptom. Ascaris lumbricoides, a type of roundworm infection, may cause a variety of symptoms related to intestinal obstruction but not the "coffee ground" emesis associated with the intrusive effects of Entamoeba histolytica. Thus, the correlation of abdominal symptoms and the specific type of vomit appearance aligns well with the pathology caused by Entamoeba histolytica.