To identify a disease where the breakdown (degradation) of glycoproteins is impaired. Glycoproteins are proteins with attached carbohydrate chains, and their degradation primarily occurs in lysosomes.

Here's the breakdown of the question about diseases associated with defective glycoprotein degradation:

• Step 1: Understand the question. The question asks to identify a disease where the breakdown (degradation) of glycoproteins is impaired. Glycoproteins are proteins with attached carbohydrate chains, and their degradation primarily occurs in lysosomes.

• Step 2: Analyze the options.

  • A. I-cell disease (Mucolipidosis II): This is a lysosomal storage disorder caused by a deficiency in N-acetylglucosamine-1-phosphotransferase. This enzyme is crucial for tagging lysosomal enzymes with mannose-6-phosphate, which directs them to the lysosomes. Without this tag, lysosomal enzymes are secreted instead of being delivered to the lysosomes. This leads to a widespread accumulation of various undegraded macromolecules, including glycoproteins and glycolipids, within the lysosomes. Thus, it directly involves defective glycoprotein degradation.
  • B. Sickle cell anemia: This is a genetic blood disorder affecting hemoglobin, not glycoprotein degradation.
  • C. Phenylketonuria (PKU): This is a metabolic disorder involving the inability to metabolize the amino acid phenylalanine, not glycoprotein degradation.
  • D. Tay-Sachs disease: This is a lysosomal storage disorder caused by a deficiency in beta-hexosaminidase A, leading to the accumulation of GM2 gangliosides (a type of glycolipid). While it's a lysosomal disorder, its primary defect is in glycolipid degradation, not specifically glycoprotein degradation. I-cell disease represents a more general defect in lysosomal function affecting multiple substrates, including glycoproteins.

• Step 3: Select the best fit. I-cell disease is characterized by a broad defect in lysosomal enzyme targeting, leading to the accumulation of many undegraded substances, including glycoproteins.

The final answer is $\boxed{\text{A}}$