Locally subquadrangular hyperplanes in symplectic and Hermitian dual polar spaces

In Pasini and Shpectorov (2001) [11] all locally subquadrangular hyperplanes of finite symplectic and Hermitian dual polar spaces were determined with the aid of counting arguments and divisibility properties of integers. In the present note we extend this classification to the infinite case. We prove that symplectic dual polar spaces and certain Hermitian dual polar spaces cannot have locally subquadrangular hyperplanes if their rank is at least three and their lines contain more than three points.