**Core Concept**
The Grynfeltt triangle is a clinically significant anatomical landmark in the lower back, bounded by specific muscles and a rib. It is an important area to recognize for surgical and diagnostic procedures.

**Why the Correct Answer is Right**
The Grynfeltt triangle is bounded by the 12th rib, the internal oblique muscle, and the paraspinal muscles. The external oblique muscle does not form the boundary of this triangle, which is located in the posterior abdominal wall. The internal oblique muscle plays a crucial role in the formation of the Grynfeltt triangle, and its boundary helps in identifying the location of the triangle. The paraspinal muscles also contribute to the definition of the triangle's boundaries.

**Why Each Wrong Option is Incorrect**
**Option A:** The 12th rib is actually one of the boundaries of the Grynfeltt triangle, so option A is incorrect because it mentions a muscle that is not part of the triangle's boundary.
**Option B:** The paraspinal muscles are also one of the boundaries of the Grynfeltt triangle, so option B is incorrect for the same reason as option A.
**Option C:** The internal oblique muscle is indeed a boundary of the Grynfeltt triangle, so option C is incorrect because it is actually part of the correct definition.

**Clinical Pearl / High-Yield Fact**
The Grynfeltt triangle is an important landmark for surgeons and radiologists to identify, as it is often used as a reference point for various diagnostic and therapeutic procedures in the lower back. A thorough understanding of the anatomy of this region is essential for accurate diagnosis and treatment.

**✓ Correct Answer: D. External oblique muscle**