**Core Concept**
Thrombopoietin (THPO) is a hormone that regulates platelet production. It is produced by two main organs, which play a crucial role in maintaining the balance of platelets in the blood.

**Why the Correct Answer is Right**
Thrombopoietin is produced by the liver and kidneys, where it is synthesized and released into the circulation. The liver is the primary site of THPO production, accounting for approximately 80% of total production. The kidneys produce the remaining 20%. THPO acts on the bone marrow to stimulate the production of megakaryocytes, which mature into platelets. The liver and kidneys work in tandem to maintain the delicate balance of platelet production, ensuring that the body has an adequate supply of platelets to prevent bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** The kidneys produce a portion of thrombopoietin, but they are not the primary site of production. The kidneys' role is often underestimated in the context of THPO production.
**Option B:** While the liver is the primary site of thrombopoietin production, it is not the sole source of this hormone. The kidneys also contribute to THPO production.
**Option D:** The spleen and lymph nodes are not involved in the production of thrombopoietin. They play roles in filtering the blood and immune response, but not in regulating platelet production.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that thrombopoietin production is a complex process involving multiple organs. The liver and kidneys work together to maintain the balance of platelet production, and any disruption to this process can lead to thrombocytopenia or thrombocytosis.

✓ Correct Answer: C. Liver and kidneys