Microscopic examination of a specimen shows organisms with characters of both corynebacteria and streptococci and resemble diphtheroids in stained smears. The organism is:
### Core Concept
The question is testing the student's ability to identify an organism based on its microscopic characteristics, which show features of both corynebacteria and streptococci. This requires knowledge of the morphological characteristics of different bacterial species.
### Why the Correct Answer is Right
The organism in question is likely *Corynebacterium diphtheriae*, which is a Gram-positive, rod-shaped bacterium that can exhibit both coryneform (club-shaped) and streptococcal (clustered) arrangements. The presence of metachromatic granules, which are characteristic of corynebacteria, is also a key feature of this organism. The ability of *C. diphtheriae* to produce diphtheria toxin is a significant virulence factor.
### Why Each Wrong Option is Incorrect
**Option A:** This option is incorrect because it does not match the microscopic description provided. While *Corynebacterium ulcerans* is a close relative of *C. diphtheriae*, it does not exhibit the same level of morphological variation.
**Option B:** This option is incorrect because *Streptococcus pyogenes* is a Gram-positive coccus that typically forms chains, rather than exhibiting the coryneform arrangements described.
**Option C:** This option is incorrect because *Bacillus anthracis* is a Gram-positive rod that forms terminal spores, rather than exhibiting the metachromatic granules or streptococcal arrangements described.
### Clinical Pearl / High-Yield Fact
* C. diphtheriae *can infect both humans and animals, but human-to-human transmission is the primary mode of spread. This organism is a significant public health concern due to its ability to produce diphtheria toxin, which can cause severe respiratory and cardiac complications.
### Correct Answer: C. Corynebacterium diphtheriae.