Initial site of RBC production in fetus
**Core Concept**
The initial site of red blood cell (RBC) production in the fetus is a critical aspect of hematopoiesis, which is the process of forming new blood cells. In the fetus, the primary site of RBC production is different from that in adults.
**Why the Correct Answer is Right**
The initial site of RBC production in the fetus is the yolk sac, which starts producing RBCs around 2-3 weeks of gestation. This is followed by the liver and spleen, where erythropoiesis (RBC production) continues to increase. The yolk sac is a temporary site that eventually gives way to the liver and spleen as the primary sites of RBC production. The production of RBCs in the fetus is essential for meeting the high oxygen demands of the growing embryo.
**Why Each Wrong Option is Incorrect**
* **Option A:** While the liver and spleen do play a role in RBC production in the fetus, they are not the initial site.
* **Option B:** The bone marrow becomes the primary site of RBC production in adults, but it is not the initial site in the fetus.
* **Option D:** The placenta is essential for gas exchange and nutrient transfer between the mother and fetus, but it is not a site of RBC production.
**Clinical Pearl / High-Yield Fact**
The yolk sac is a temporary site of RBC production in the fetus, and its function is replaced by the liver and spleen later in gestation. This is a critical aspect of fetal development, and understanding the sites of RBC production can help in the diagnosis and management of hematological disorders in newborns.
**Correct Answer: C. Yolk sac**