The correct answer is c- Absence of IgM.

CSF protein electrophoresis differs from serum protein electrophoresis in terms of the protein composition. CSF protein electrophoresis shows a different protein profile due to the presence of proteins that are synthesized in the central nervous system (CNS) and those that have crossed the blood-brain barrier (BBB) from the serum.

CSF protein electrophoresis typically shows a small albumin band, which is not as prominent as in serum protein electrophoresis. This is because albumin is synthesized in the liver and does not cross the BBB easily.

Tau protein is a specific protein that is found in neurons and is not present in serum. Therefore, it is not detected in serum protein electrophoresis. However, in CSF protein electrophoresis, tau protein can be detected if there is neuronal damage or degeneration, as it leaks into the CSF.

IgM is a large immunoglobulin that is present in serum but is absent in CSF. This is due to the presence of the BBB, which prevents large molecules like IgM from crossing into the CNS.

Alpha-1-antitrypsin (AAT) and alpha-2-macroglobulin (A2M) are two alpha bands that are present in both serum and CSF protein electrophoresis. However, the relative proportions of these two proteins are different in serum and CSF. In serum, AAT is more abundant than A2M, while in CSF, A2M is more abundant than AAT.

In summary, CSF protein electrophoresis differs from serum protein electrophoresis in terms of the presence or absence of certain proteins. CSF protein electrophoresis typically shows a small albumin band, the presence of tau protein, the absence of IgM, and a different ratio of AAT and A2M.