Final answer:
Erythropoietin (EPO) is a glycoprotein hormone chiefly produced by the kidney, responsible for stimulating the production of red blood cells in the bone marrow in response to low oxygen levels. Therefore, the correct answer is 2.
Step-by-step explanation:
Erythropoietin (EPO) is a glycoprotein hormone that is primarily produced by the kidney. When there is a decrease in oxygen levels or blood flow to the kidneys, EPO is secreted by the interstitial fibroblast cells. This hormone plays a critical role in erythropoiesis, the process of red blood cell production, by stimulating the bone marrow to produce more erythrocytes, which are the main cellular component of blood.
Although a small percentage of EPO is produced by the liver, the vast majority, approximately 85%, is produced by the kidneys. Conditions such as high altitude, aerobic exercise, blood loss, or disease can trigger the kidneys to increase EPO production to compensate for increased oxygen demands or replenish lost erythrocytes. Without sufficient EPO, as in cases of renal failure, individuals may suffer from anemia, which can significantly impair the body's oxygen delivery system and be life-threatening.
Therefore, the correct answer to the student's question is 2) Kidney.