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A 6-year-old boy presents with a rash that started as a superficial accumulation of several small vesicles on his legs below the knees. He is afebrile and appears normal other than these lesions. The child lives in the suburbs and often plays outside in the local woods in short pants. The lesions are honey-brown and crusted, with an erythematous base, and are in various stages of crusting and openness. What is the most important laboratory test to distinguish between the genera of potential causative agents of this condition?
Options:
A. CAMP test
B. Catalase test
C. Coagulase test
D. ELEK test
E. Gram's stain
F. Oxidase test
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The correct answer is B. This is likely a case of Streptococcus pyogenes impetigo because of the description of honey-crusted lesions, but the other major cause of impetigo is Staphylococcus aureus, and these two genera are distinguished on the basis of the catalase test.
CAMP test (choice A) is the test used to distinguish the group B streptococci from other streptococci, and would not be useful here.
The coagulase test (choice C) is the test used to distinguish Staphylococcus aureus from all other
staphylococci, and would not distinguish between the genera of agents causing impetigo.
The ELEK test (choice D) is the immunodiffusion test used to identify toxin-producing strains of
Corynebacterium diphtheriae.
Gram's stain (choice E) would not be useful, as both of the most common agents of impetigo are
gram-positive cocci.
The oxidase test (choice F) is the test used to identify Neisseria and would not be useful here.
