Okay, let me tackle this question. The question is about the active transport of calcium being regulated by a substance synthesized in the kidneys. The options are A, B, C, D, but they aren't provided here. Wait, the user mentioned the correct answer is [Letter]. [Answer Text], but in the example, the correct answer line is left blank. Hmm, maybe the correct answer is D. 1,25-Dihydroxycholecalciferol?

First, I need to recall the hormones involved in calcium regulation. The main ones are parathyroid hormone (PTH), calcitriol (active form of vitamin D), and calcitonin. Since the question specifies that the substance is synthesized in the kidneys, that points towards calcitriol. PTH is produced by the parathyroid glands, not the kidneys. Calcitriol, the active form of vitamin D, is synthesized in the kidneys through the action of 1α-hydroxylase enzyme, converting 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D (calcitriol). This active form increases calcium absorption in the intestines and promotes calcium reabsorption in the kidneys, aiding in active transport.

The other options would be things like PTH, calcitonin, or maybe others like aldosterone or ADH. But since the question mentions the kidneys as the site of synthesis, PTH is produced in the parathyroid glands, so that's out. Calcitonin is from the thyroid. So the correct answer must be calcitriol. Let me make sure: the active transport of calcium in the kidneys and intestines is indeed regulated by calcitriol. The key here is the synthesis in the kidneys. So the answer is D. 1,25-Dihydroxycholecalciferol.

Now, the explanation. The core concept is calcium homeostasis and the role of calcitriol. The correct answer is because calcitriol is synthesized in the kidneys via 1α-hydroxylase. The wrong options would be PTH (parathyroid), calcitonin (thyroid), or maybe others. Clinical pearl: Remember that the kidneys are the site where vitamin D is activated to its active form, which is crucial for calcium absorption and reabsorption. High-yield fact: PTH stimulates the production of calcitriol in the kidneys, creating a feedback loop for calcium regulation.

**Core Concept**
Calcium homeostasis is maintained by hormones regulating intestinal absorption, renal reabsorption, and bone remodeling. The kidneys synthesize **1,25-dihydroxycholecalciferol** (calcitriol), the active form of vitamin D, to enhance calcium absorption and reabsorption.

**Why the Correct Answer is Right**
**1,25-Dihydroxycholecalciferol** is produced in the kidneys via the enzyme **1α-hydroxylase**, which hydroxylates 25-hydroxyvitamin D (produced in the liver). It binds to **vitamin D receptors** in the intestines and kidneys, increasing **TRPV6 channel activity** in the gut (for calcium absorption) and **Na⁺/Ca²⁺ exchangers** in the renal tubules (for

✓ Correct Answer: D. 1, 25-di OH-Cholecalciferol