**Core Concept**
The kidneys play a crucial role in regulating various bodily functions, including fluid balance, electrolyte balance, and blood pressure. They produce several hormones and substances that contribute to these processes.

**Why the Correct Answer is Right**
The kidneys produce several essential substances, including erythropoietin (EPO), renin, and 1,25-dihydroxyvitamin D. EPO stimulates red blood cell production, renin initiates the renin-angiotensin-aldosterone system (RAAS) to regulate blood pressure, and 1,25-dihydroxyvitamin D promotes calcium absorption in the gut. However, the kidneys do not produce calcitonin.

**Why Each Wrong Option is Incorrect**
**Option A:** Erythropoietin (EPO) is indeed formed by the kidneys, making this option incorrect.
**Option B:** Renin is also produced by the kidneys, particularly in the juxtaglomerular cells, making this option incorrect.
**Option C:** 1,25-dihydroxyvitamin D is synthesized in the kidneys from its precursor, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
Remember that the kidneys regulate fluid balance, electrolyte balance, and blood pressure through the production of renin, EPO, and 1,25-dihydroxyvitamin D. However, they do not produce calcitonin, which is produced by the thyroid gland.

**Correct Answer:** D. Calcitonin.