**Question:** Androgen binding protein is secreted by the following structure:

A. Leydig cells
B. Sertoli cells
C. Granulosa cells
D. Erythrocytes

**Core Concept:** Androgen binding protein (ABP) is a transport protein that helps in the delivery of biologically active androgens (male sex hormones) to target tissues. It plays a crucial role in maintaining the balance between the circulating androgen levels and their utilization at the target sites.

**Why the Correct Answer is Right:** Androgen binding protein is primarily secreted by Sertoli cells, which are responsible for supporting the developing germ cells (spermatogenesis) in the testes. The correct answer is B, Sertoli cells, because they synthesize and secrete ABP to facilitate the transport of biologically active androgens, such as testosterone, to the target tissues.

**Why Each Wrong Option is Incorrect:**

A. Leydig cells: These cells are responsible for the synthesis of testosterone, not ABP. Although Leydig cells do interact with ABP, they do not secrete it.

C. Granulosa cells: Granulosa cells are involved in the production of sex hormones in the female reproductive system, specifically in the ovaries. They are not responsible for ABP secretion.

D. Erythrocytes: Erythrocytes are red blood cells and do not secrete any proteins, let alone androgen binding protein.

**Clinical Pearl:** The understanding of androgen binding protein and its role in testosterone transport is crucial for the proper functioning of male reproductive system. ABP contributes to the bioavailability of testosterone at the target tissues like Leydig cells, Sertoli cells, and germ cells, ensuring the normal development and function of male reproductive system.