## Core Concept
Corynebacterium diphtheriae is a Gram-positive, rod-shaped bacterium that causes diphtheria, a serious infection that affects the mucous membranes of the throat and nose. The bacteria produce a toxin that can cause severe respiratory and cardiac complications. Understanding the characteristics of C. diphtheriae is crucial for diagnosing and managing diphtheria.

## Why the Correct Answer is Right
The correct answer, ., implies that one of the statements provided about C. diphtheriae is not true. To assess this, we need to evaluate each option based on known facts about C. diphtheriae. Typically, C. diphtheriae is known for producing a toxin encoded by a bacteriophage (a virus that infects bacteria), and its identification involves specific staining techniques and culture methods.

## Why Each Wrong Option is Incorrect
- **Option A:** Typically, C. diphtheriae can be identified by its characteristic "pick a back" or palisade arrangement of cells and by staining techniques such as Albert's stain, which shows metachromatic granules. If option A states a true characteristic, it would not be the correct answer.
- **Option B:** If option B describes a true property of C. diphtheriae, such as being a causative agent of diphtheria or producing diphtheria toxin, then it would not be the correct answer.
- **Option C:** Similarly, if option C accurately describes a feature of C. diphtheriae, it would not be the correct choice.
- **Option D:** Given that ., the statement in option D must be false regarding C. diphtheriae.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the pathogenicity of C. diphtheriae is primarily due to its ability to produce diphtheria toxin, which is encoded by a gene carried by a bacteriophage (a virus that infects bacteria). This toxin can cause local tissue damage and systemic effects, including myocarditis and neuropathy.

## Correct Answer: D.