Discoid lupus : Pathognomonic or a key sign? A subtle difference

Discoid lupus: Pathognomonic or a Key sign?

Pathognomonic is a term, often used in medicine, that means characteristic for a particular disease. A pathognomonic sign is a particular sign whose presence means that a particular disease is present beyond any doubt (Wikipedia).

There is another term “Key sign” used to differentiate between three similar entities inside the same pattern. A pathognomonic sign can be a Key sign, but the opposite is not true. What is clear is that a Key sign is present in one entity, but not in the other two in the same pattern.