Which natural toxin is associated with scallops and mussels leading to diarrhea and abdominal pain?

Diarrhetic shellfish poisoning is the correct association because this natural toxin specifically involves certain species of shellfish, including scallops and mussels, as vectors for its effects. It is caused by the consumption of shellfish that have accumulated toxins produced by harmful algae, primarily those in the dinoflagellate family. The symptoms, which include diarrhea and abdominal pain, arise when people ingest contaminated shellfish.

Understanding this context helps clarify why this specific option is chosen. Diarrhetic shellfish poisoning is characterized by gastrointestinal symptoms, which perfectly align with the symptoms mentioned in the question. The prevalence of this toxin particularly in scallops and mussels makes it a significant concern in food safety regarding these shellfish types.