In cooked-meat (Robertson) medium, blackening of the meat particles is caused by which organism among the options listed? Select the species classically associated with pronounced blackening due to sulfite reduction and related reactions.

Introduction:
Cooked-meat (Robertson) medium is used to culture anaerobes. Certain clostridia produce blackening of the meat due to reduction of sulfite and formation of iron sulfide, a useful visual clue during preliminary workup.

Given Data / Assumptions:

• We compare C. histolyticum, C. perfringens, and C. tetani.
• Blackening refers to obvious discoloration of meat particles in the medium.
• We seek the species most classically associated with this reaction among listed choices.

Concept / Approach:
C. perfringens commonly produces strong blackening in cooked-meat media because of sulfite reduction and subsequent iron sulfide deposition. While some proteolytic clostridia can darken media, C. tetani is not a reliable blackener. Among the provided choices, C. perfringens best fits the classic description.

Step-by-Step Solution:
Step 1: Recall characteristic reactions of common clostridia in cooked-meat medium. Step 2: Note that C. perfringens frequently causes prominent blackening. Step 3: Evaluate other options: C. tetani typically does not blacken strongly; C. histolyticum varies and is less classically cited for blackening in this context. Step 4: Choose “Clostridium perfringens.”

Verification / Alternative check:
Parallel findings such as stormy clot in milk and lecithinase positivity (Nagler's reaction) support identification of C. perfringens from primary culture when seen together with blackening.

Why Other Options Are Wrong:
C. histolyticum – proteolytic but not the textbook example for meat blackening among these options.

C. tetani – generally poor blackening; identification relies on drumstick spores and neurotoxin association.

All of these – incorrect because the reaction is not typical for every listed species.

None of the above – incorrect since C. perfringens fits.

Common Pitfalls:
Assuming any anaerobe that grows will blacken the medium; blackening is a specific chemical result and varies by species and conditions.

Final Answer:
Clostridium perfringens.