**Question:** All are true about gastric Ca Except

A. Gastric calcium (Ca) plays a crucial role in maintaining the integrity of the gastric mucosa and regulating gastric acid secretion.
B. Gastric Ca is involved in the regulation of smooth muscle contraction and relaxation in the gastrointestinal tract.
C. Gastric Ca is essential for the proper development and functioning of the nervous system.
D. Gastric Ca levels are regulated by vitamin D3, which is not true for calcium in the stomach.

**Correct Answer:** D. Gastric Ca levels are regulated by intrinsic factors such as parathyroid hormone (PTH) and calcitonin, rather than vitamin D3.

**Core Concept:**
Gastric calcium (Ca) is a vital element in the gastrointestinal system and plays a significant role in maintaining gastric mucosal integrity and regulating gastric acid secretion. In the stomach, calcium ions (Ca²⁺) bind to intrinsic factor (IF) and the vitamin B12-IF complex, forming a complex that aids in the absorption of vitamin B12 in the small intestine.

**Why the Correct Answer is Right:**
The correct answer (D) highlights the specific regulatory mechanisms of gastric Ca levels. In contrast to the general regulation of Ca levels by vitamin D3, intrinsic factors (parathyroid hormone, PTH, and calcitonin) control the gastric Ca balance. Intrinsic factors are released by the parathyroid gland, which influences Ca²⁺ reabsorption in the kidney and promotes Ca²⁺ release from the bones, thus affecting overall Ca²⁺ levels in the blood.

**Why Each Wrong Option is Incorrect:**

A. Gastric Ca plays a vital role in maintaining gastric mucosal integrity and regulating gastric acid secretion, which is true. However, the regulation of gastric Ca levels is not related to vitamin D3, but rather intrinsic factors, as explained above.

B. Gastric Ca is involved in the regulation of smooth muscle contraction and relaxation in the gastrointestinal tract, which is partially true. However, the primary regulatory mechanism for gastric Ca is intrinsic factors, not vitamin D3.

C. Gastric Ca is essential for the proper development and functioning of the nervous system, which is incorrect in this context. The explanation provided focuses on the role of gastric Ca in the gastrointestinal system, not the nervous system.

**Clinical Pearl:**
In clinical practice, understanding the regulation of gastric Ca levels is crucial for diagnosing and treating disorders like hypocalcemia, hypercalcemia, or vitamin B12 deficiency. Monitoring gastric Ca levels alongside vitamin B12 levels can provide valuable information about the overall calcium homeostasis in the body.

In summary, the regulation of gastric calcium levels is primarily influenced by intrinsic factors (parathyroid hormone and calcitonin) rather than vitamin D3, which regulates calcium levels in the blood and bone. The heart, nervous system, and muscles rely on calcium homeostasis managed by vitamin D3, PTH, and calcitonin.