**Core Concept**
Erythropoietin (EPO) is a hormone crucial for regulating red blood cell production in the body. It is produced in response to low oxygen levels (hypoxia) and stimulates the bone marrow to increase erythropoiesis.

**Why the Correct Answer is Right**
EPO is secreted by interstitial cells in peritubular capillaries of the kidney, specifically in the renal cortex. These cells, also known as peritubular interstitial fibroblasts, respond to hypoxia by producing EPO. The production of EPO is mediated by the hypoxia-inducible factor (HIF), which is stabilized under low oxygen conditions. HIF then activates the transcription of the EPO gene, leading to the secretion of EPO into the bloodstream. EPO then acts on the bone marrow to stimulate the production of red blood cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Juxtaglomerular cells are involved in the regulation of renin release and are not responsible for EPO production.

**Option B:** Proximal convoluted tubule (PCT) cells are primarily involved in the reabsorption of nutrients and the secretion of substances like uric acid, but they are not the primary source of EPO.

**Option D:** The capillaries of the glomerulus are involved in the filtration of blood and the formation of urine, but they are not responsible for EPO production.

**Clinical Pearl / High-Yield Fact**
EPO production is suppressed in conditions of high oxygen levels, such as high altitude or carbon monoxide poisoning, leading to a decrease in red blood cell production. This is an important consideration in patients with chronic kidney disease or those undergoing chemotherapy.

**✓ Correct Answer: C. Interstitial cells in peritubular capillaries**