A man developed an allergic reaction to an unknown food. his doctor asked him about a number of foods that often causes allergies, and the man replied that he had eaten coconut, chocolate, nuts, milk products, shellfish, peppers, eggs, and wheat products prior to

It is not nuts, milk, chocolate, eggs, or wheat. We can only assume he has consumed those before hand and had no symptoms. Peppers are also unlikely. Now, we only have shellfish and coconut. Now, shellfish is more common than coconut, so based on that, we can only assume the man is allergic to shellfish.