A 33-year-old male presents with an itchy rash over his
scalp and on his face. The rash came on quite suddenly is very severe, and
extensive and is not responding to simple local treatment that he was given by
On examination, on the scalp there are widespread thick
adherent crusts. The skin shows greasy scaling over red inflamed skin.
This involves the forehead, the eyebrows, the lash line, the nasolabial
folds, the beard, and the post auricular skin.
In this patient which of the following tests is most
likely to determine the underlying cause of the disease?