The test likely to help in diagnosis of a patient who presents with an itchy annular plaque on the face is –
Gram's stain
Potassium hydroxide mount
Tissue smear
Wood's lamp examination
The clinical picture is suggestive of a fungal infection most likely Tinea fascitis, KOH preparation is performed on scaling skin lesion when fungal etiology is suspected. Microscope examination after KOH digestion will show fungal hyphae.