Final answer:
A mutation that disrupts the ability of an animal cell to add polysaccharide modifications to proteins would most likely cause defects in the Golgi apparatus and the extracellular matrix.
Step-by-step explanation:
When an animal cell is unable to add polysaccharide modifications to proteins, it indicates a defect in the Golgi apparatus. The Golgi apparatus is responsible for the modification and sorting of proteins, including the addition of polysaccharide chains in a process called glycosylation.
These modified proteins are then packaged into vesicles for transport to various cellular locations, including the extracellular matrix. Therefore, a mutation in the ability to add polysaccharide modifications would most likely cause defects in both the Golgi apparatus and the extracellular matrix.