Toxin susceptibility testing: Which test historically indicated a patient’s cutaneous susceptibility to streptococcal pyrogenic (erythrogenic) exotoxin associated with scarlet fever?

Introduction / Context:
Before modern molecular diagnostics, toxin susceptibility tests were used to infer immunity to specific bacterial exotoxins. For streptococcal scarlet fever, a historical intradermal test assessed host sensitivity to pyrogenic (erythrogenic) exotoxin.

Given Data / Assumptions:

• “Schick test” evaluates diphtheria toxin susceptibility, not streptococcal exotoxin.
• ASO measures antibodies to streptolysin O, reflecting past infection, not toxin susceptibility.
• The item labeled “Disk test” refers to the classic “Dick test,” the historical skin test for susceptibility to streptococcal erythrogenic toxin.

Concept / Approach:

The Dick test involved intradermal injection of diluted streptococcal exotoxin with a control injection on the opposite arm. A local erythematous response indicated susceptibility; absence suggested neutralizing antibodies. Although obsolete, the concept distinguishes between toxin susceptibility testing and antibody titer assays.

Step-by-Step Solution:

Verification / Alternative check:

Medical history texts document the Dick test use when scarlet fever was common. Positive skin reactions correlated with clinical susceptibility.

Why Other Options Are Wrong:

Schick test is specific to Corynebacterium diphtheriae toxin.

ASO does not test susceptibility; it detects antibodies post-infection.

“Precipitation test” is nonspecific and not the named historical assay for this toxin.

Common Pitfalls:

Confusing similar-sounding “Schick” and “Dick” tests; remember “Dick” for scarlet fever toxin, “Schick” for diphtheria toxin.

Final Answer:

Disk test