What is the single most useful investigation?

A 45-year-old man has developed an annular rash with a scaly edge on his thigh. The rash has been spreading. He has some general aches and pains.

What is the single most useful investigation?
A . ANA
B . Skin scrap for mycology
C . Skin swab for bacteria
D . Biopsy lesion
E . Lyme antibodies

Answer: E

Explanation:

Explanation Spreading annular rash suggests erythema migrans of lyme disease.