True about calcium reabsorption in kidney ?
**Core Concept**
Calcium reabsorption in the kidney is a crucial process that helps maintain serum calcium levels within a narrow range. The majority of calcium reabsorption occurs in the proximal convoluted tubule (PCT) and the distal convoluted tubule (DCT), with the PCT being the primary site.
**Why the Correct Answer is Right**
Calcium reabsorption in the PCT is facilitated by the sodium-calcium exchanger (NCX) and the calcium-binding protein calbindin-D28k. The NCX transports calcium ions out of the lumen and into the cell in exchange for sodium ions. Calbindin-D28k binds to calcium ions, facilitating their transport across the basolateral membrane into the bloodstream. In the DCT, calcium reabsorption is regulated by the parathyroid hormone (PTH) and vitamin D. PTH increases calcium reabsorption by stimulating the production of 1,25-dihydroxyvitamin D, which enhances the expression of the calcium transporter protein.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not specify the primary site of calcium reabsorption. While the distal convoluted tubule (DCT) does play a role in calcium reabsorption, the proximal convoluted tubule (PCT) is the primary site.
**Option B:** This option is incorrect as it does not mention the sodium-calcium exchanger (NCX), which is a key player in calcium reabsorption in the PCT.
**Option C:** This option is incorrect as it does not mention calbindin-D28k, which is essential for calcium transport across the basolateral membrane in the PCT.
**Option D:** This option is incorrect as it does not specify the role of vitamin D in regulating calcium reabsorption in the DCT.
**Clinical Pearl / High-Yield Fact**
The kidneys reabsorb approximately 99% of filtered calcium, making the kidneys a critical regulator of serum calcium levels. Disruptions in calcium reabsorption can lead to disorders such as hypercalcemia or hypocalcemia.
**Correct Answer: C. The sodium-calcium exchanger (NCX), calbindin-D28k, and vitamin D all play crucial roles in calcium reabsorption in the kidney.