Calcium absorption is increased by –
**Core Concept**
Calcium absorption is a complex process influenced by various factors, including dietary components, hormonal regulation, and the presence of certain minerals. The primary site of calcium absorption is the small intestine, where calcium is absorbed through a process involving the activation of vitamin D and the presence of specific transport proteins.
**Why the Correct Answer is Right**
The correct answer involves the activation of vitamin D, which plays a crucial role in enhancing calcium absorption. When vitamin D is activated, it binds to the vitamin D receptor (VDR) in the enterocytes of the small intestine, promoting the expression of the calcium-binding protein calbindin. This protein facilitates the transport of calcium ions across the enterocyte membrane, increasing calcium absorption. The presence of magnesium also enhances calcium absorption by stimulating the release of parathyroid hormone-related protein (PTHrP), which in turn activates the VDR.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while vitamin K is essential for blood clotting, it does not directly influence calcium absorption.
**Option B:** This option is incorrect because while magnesium is essential for many bodily functions, it is not the primary factor that increases calcium absorption in the small intestine.
**Option C:** This option is incorrect because while phosphorus is an essential mineral, it does not directly influence calcium absorption in the small intestine.
**Clinical Pearl / High-Yield Fact**
One key clinical correlation to remember is that patients with vitamin D deficiency or insufficiency are at increased risk of osteoporosis and fractures. Regular monitoring of vitamin D levels and supplementation when necessary is essential to maintain optimal bone health.
**Correct Answer: D. Vitamin D**