This suggestion is incorrect and should be changed back. The comma in this phrase is introducing a synonymous phrase that acts as an appositive. The use of the comma in this phrase signals equivalence rather than alternation. The removal of the comma could confuse learners into thinking that “tumor” and “swelling” are two binary concepts, when they are the same cardinal sign of inflammation. Even with the “One cardinal sign” at the beginning, the grammatical concept still holds.