Equine Image Quiz: A colt with an impacted small intestine

A 11-month-old Thoroughbred colt was evaluated for acute signs of severe abdominal discomfort. The colt had a history of poor growth and a rough coat. A fecal egg count had revealed a large number of intestinal parasite eggs (see photo above), so he had received an anthelmintic 24 hours before the onset of the clinical signs of colic.

An abdominal ultrasonographic examination revealed a small intestinal impaction due to an accumulation of dead parasites (see arrow in the photo below).

Which parasite is most likely involved in this case? 

a) Strongyloides westeri

b) Gasterophilus intestinalis

c) Parascaris equorum

d) Strongylus vulgaris

e) Oxyuris equi