**Core Concept**
The posterior relations of the kidney are critical to understanding the anatomy of the retroperitoneal space. The kidneys are bean-shaped organs located in the lumbar region, and their posterior surface is related to several structures that are essential for their function and protection.

**Why the Correct Answer is Right**
The correct answer is related to the fact that the psoas major muscle is a key posterior relation of the kidney. The psoas major muscle originates from the transverse processes of the lumbar vertebrae and inserts into the lesser trochanter of the femur. It is a long and flat muscle that runs along the posterior aspect of the abdominal cavity, and it is closely related to the posterior surface of the kidney. The psoas major muscle plays a crucial role in flexion of the hip joint and is also involved in the stability of the lumbar spine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the diaphragm is actually an anterior relation of the kidney, not a posterior relation. The diaphragm is a dome-shaped muscle that separates the chest cavity from the abdominal cavity, and it is closely related to the anterior surface of the kidney.
* **Option B:** This option is incorrect because the adrenal gland is actually an anterior relation of the kidney, not a posterior relation. The adrenal gland is a small endocrine gland that is located on top of the kidney and produces hormones such as adrenaline and cortisol.
* **Option D:** This option is incorrect because the quadratus lumborum muscle is actually a lateral relation of the kidney, not a posterior relation. The quadratus lumborum muscle is a flat muscle that runs along the lateral aspect of the abdominal cavity, and it is closely related to the lateral surface of the kidney.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the posterior relations of the kidney are closely related to the lumbar spine and the psoas major muscle. The psoas major muscle can be palpated during a physical examination, and its tenderness can indicate lumbar spine pathology or kidney disease.

**Correct Answer:** D. Quadratus lumborum muscle.