First, you are welcome to submit your own question. It's harder than it looks and this seems to be a straightforward question.
Second, a primary key does not include a clustered index. They are not related in any way. A primary key can be a clustered index, but it can also be a non clustered index. The reverse is true as well.
Clustered or nonclustered indexes are determined for performance reasons. These are physical constructs that determine how the index is structured.
A primary key is a logical construct, that lets the designer/developer know which column(s) uniquely identify a row. This is implemented in SQL Server with a unique index, but it can be clustered or nonclustered.