5. If the vertices of a regular tetrahedron $P Q M N$ are on the faces of a given tetrahedron $A B C D$, with one point on each face, then, ( ).
(A) When the tetrahedron $A B C D$ is a regular tetrahedron, there are infinitely many regular tetrahedrons $P Q M N$, otherwise, there is only one regular tetrahedron $P Q M N$
(B) When the tetrahedron $A B C D$ is a regular tetrahedron, there are infinitely many regular tetrahedrons $P Q M N$, otherwise, no regular tetrahedron $P Q M N$ exists
(C) When the tetrahedron $A B C D$ has three pairs of equal opposite edges, there are infinitely many regular tetrahedrons $P Q M N$, otherwise, there is only one regular tetrahedron $P Q M N$
(D) For any tetrahedron $A B C D$, there are infinitely many regular tetrahedrons $P Q M N$
5.D.

In the tetrahedron $ABCD$, construct a sufficiently small regular tetrahedron $PQMN$ inside it, such that it can rotate freely within the tetrahedron, with the distances from each vertex of the regular tetrahedron $PQMN$ to the corresponding face being different. Through the vertex closest to each face, draw a plane parallel to that face, and the 4 planes intersect to form a tetrahedron $A'B'C'D'$. Then, each face of the tetrahedron $A'B'C'D'$ contains exactly one vertex of the regular tetrahedron $PQMN$, and the tetrahedron $A'B'C'D'$ is similar to the tetrahedron $ABCD$. By scaling appropriately, make the tetrahedron $A'B'C'D'$ congruent to the tetrahedron $ABCD$, then each face of the tetrahedron $ABCD$ contains exactly one vertex of the regular tetrahedron $PQMN$. Since $PQ$ can have infinitely many orientations, the answer is (D).