**Core Concept**
Paneth cells are specialized epithelial cells located in the crypts of Lieberkühn in the small intestine, playing a crucial role in maintaining the gut microbiome. They produce and secrete various substances to regulate the intestinal environment, promote gut health, and prevent infection.

**Why the Correct Answer is Right**
Paneth cells produce and secrete antibacterial peptides, such as defensins and lysozyme, which have antimicrobial properties. These peptides help to eliminate pathogenic bacteria, maintain the balance of the gut microbiome, and prevent the overgrowth of harmful microorganisms. The secretion of antibacterial substances by Paneth cells is essential for maintaining the integrity of the intestinal mucosa and preventing infection.

**Why Each Wrong Option is Incorrect**
**Option B:** Lipase is an enzyme involved in the breakdown of fats, primarily produced by the pancreas and secreted into the small intestine. It is not secreted by Paneth cells.
**Option C:** Maltase is an enzyme that breaks down maltose into glucose, primarily produced by the small intestine and salivary glands. It is not secreted by Paneth cells.
**Option D:** Secretin is a hormone produced by the duodenum in response to acidic chyme from the stomach, stimulating the pancreas to secrete bicarbonate-rich fluid. It is not secreted by Paneth cells.

**Clinical Pearl / High-Yield Fact**
Paneth cells are a classic example of the gut's immune system, demonstrating the importance of the gut-liver axis in maintaining overall health. Dysfunction of Paneth cells has been linked to various gastrointestinal disorders, including inflammatory bowel disease and intestinal infections.

**✓ Correct Answer: A. Antibacterial substance**