Nagler's reaction (lecithinase test on egg-yolk agar) is especially useful for the laboratory identification of which Clostridium species? Select the organism for which a positive Nagler's reaction is classically diagnostic.

Introduction:
Nagler's reaction detects lecithinase (alpha-toxin/PLC) activity on egg-yolk agar. A demonstrable zone of opacity inhibited by specific antitoxin is a classic hallmark for one important clostridial pathogen.

Given Data / Assumptions:

• We are dealing with a plate-based biochemical test.
• The key analyte is lecithinase activity producing opaque precipitation.
• Antitoxin neutralization can confirm specificity.

Concept / Approach:
Clostridium perfringens produces a potent lecithinase (alpha toxin) that hydrolyzes lecithin in egg yolk, creating an opaque zone. Neutralization by antitoxin on a split plate strengthens identification, making this a signature test for C. perfringens.

Step-by-Step Solution:
Step 1: Recall what Nagler's reaction measures: lecithinase activity. Step 2: Identify which clostridium reliably gives a strong positive: C. perfringens. Step 3: Choose “Clostridium perfringens.”

Verification / Alternative check:
C. perfringens also shows double-zone hemolysis on blood agar and may produce a stormy clot in milk, providing multiple converging clues when read together.

Why Other Options Are Wrong:
C. tetani – neurotoxin producer; not lecithinase-based identification.

C. botulinum – neurotoxin producer; not characterized in routine labs by a strong Nagler's reaction.

C. difficile – identified by toxin assays and selective media, not by lecithinase precipitation.

None of the above – incorrect because C. perfringens is correct.

Common Pitfalls:
Confusing Nagler's reaction with stormy clot reaction or hemolysis patterns. Each test targets a different property and should be interpreted in context.

Final Answer:
Clostridium perfringens.