Final answer:
The mucoprotein forming the matrix of a hyaline cast is the Tamm-Horsfall protein, produced by renal tubular cells and comprising the primary component of these casts in the kidney's ground substance.
Step-by-step explanation:
The mucoprotein that forms the matrix of a hyaline cast is called C. Tamm-Horsfall protein. Hyaline casts are formed from a secretion of the tubular cells of the kidney. This protein is synthesized by the renal tubular cells and constitutes the primary protein component of the matrix of these casts. The composition of the ground substance in which these hyaline casts are found includes proteoglycans and other substances. Proteoglycans are composed of a protein core with polysaccharide branches, contributing to the viscous and clear nature of the ground substance.