Tetralogy of Fallot is a significant and complex congenital heart disease. It consists of four heart malformations. So if a patient is described to have TOF, it means that she/he has all those four malformations together.
However, the separate malformations are already present on the diagnoses list, as separate entities. From a data-modeling perspective, it’s a redundancy on the factors (malformations, diseases) list. This leads to problems with interpretation. As the VSD is one of TOF’s components, is already present on the diseases list, and users are allowed to enter both VSD and TOF diagnoses, there are patients with all four combinations in the database.