(C) tells us first that (1) there were some, say 13, people who ate contaminated food. Immediately after, they summon our attention with the detail that (2) those 13 people and no one else were allergic to MSG.
What's happening there is that after we read (1), we think the discrepancy is resolved, that we now understand why they're ill. But, wait. Who are these 13 people? Did they end up ill? We don't know. All we know is that they ate contaminated food and that they're allergic to MSG.
Why are we told about the MSG allergy? Honestly, I don't have a clue. Maybe the writers are making a bait of an alternative explanation? But that shouldn't work because we learned from (1) that there was contaminated food. Likely just a mish-mash of ideas to get our brain to trip.