Peptidoglycan present in the cartilage is

The correct answer is chondroitin.

Let us understand the composition of cartilage and analyze the given options step-by-step:
1. Cartilage Composition: Cartilage is a specialized, resilient connective tissue. It consists of cells called chondrocytes embedded in an extracellular matrix. This matrix is rich in collagen fibers, elastic fibers, and ground substance.
2. Ground Substance & Proteoglycans: The ground substance of cartilage is highly rich in proteoglycans (historically referred to or closely associated with peptidoglycans/mucopolysaccharides). The major glycosaminoglycan present in cartilage matrix is chondroitin sulfate. It provides structural support, elasticity, and resistance to compression.
3. Evaluating the Options:
- chondroitin: As established, chondroitin (specifically chondroitin sulfate) is the key glycosaminoglycan/peptidoglycan constituent of cartilage.
- ossein: Ossein is the organic protein matrix found specifically in bones, not cartilage.
- cartilagin: This is a generic or obsolete term sometimes confused with cartilage proteins but is not the standard biochemical peptidoglycan/mucopolysaccharide of cartilage.
- casein: Casein is a phosphoprotein found in milk, completely unrelated to skeletal tissues.

Therefore, the major peptidoglycan (mucopolysaccharide) constituent present in cartilage is chondroitin.