**Core Concept**
Calcium absorption in the intestine is a complex process involving various factors, including dietary components, pH, and other nutrients. In this context, understanding how different substances affect calcium absorption is crucial for maintaining optimal calcium levels in the body.

**Why the Correct Answer is Right**
Phytic acid, also known as phytate, is a naturally occurring compound found in high amounts in plant-based foods such as legumes, cereals, and nuts. It binds to minerals, including calcium, magnesium, and iron, forming insoluble complexes that reduce their bioavailability. In the small intestine, phytic acid inhibits the activity of enzymes involved in calcium absorption, such as alkaline phosphatase, and also chelates calcium ions, making them unavailable for absorption. This leads to decreased intestinal absorption of calcium.

**Why Each Wrong Option is Incorrect**
**Option A:** Proteins do not decrease calcium absorption; in fact, they can enhance it by increasing the secretion of gastric acid and pancreatic enzymes, which aid in calcium solubilization. Additionally, some proteins, such as casein, can bind to calcium and increase its absorption.
**Option B:** Lactose, a sugar found in milk, actually increases calcium absorption by providing energy for the epithelial cells lining the small intestine and promoting the secretion of bile salts, which enhance calcium solubilization.
**Option D:** Acidity in the stomach, created by the presence of hydrochloric acid, actually increases calcium absorption by converting calcium carbonate into soluble calcium chloride.

**Clinical Pearl / High-Yield Fact**
Phytic acid can be reduced through food processing, such as soaking, sprouting, or germinating legumes and grains, which can increase their bioavailability of minerals, including calcium.

**✓ Correct Answer: C. Phytic Acid**