## Core Concept
The question tests the ability to identify bacteria that can or cannot ferment lactose, a key characteristic used in microbiological classification. Lactose fermentation is a critical test in differentiating among various bacterial species, particularly in the Enterobacteriaceae family.

## Why the Correct Answer is Right
The correct answer, **Shigella**, is known for being lactose nonfermenters, but some species may exhibit late lactose fermentation. However, in general, **Shigella** species are considered lactose nonfermenters. Other lactose nonfermenters include **Salmonella** and **Proteus**. The ability to ferment lactose is an important diagnostic tool; for instance, **E. coli** typically ferments lactose.

## Why Each Wrong Option is Incorrect
* **Option A:** If this option represents a lactose fermenter, such as **E. coli**, then it is incorrect because **E. coli** is known for fermenting lactose.
* **Option B:** If this option represents another lactose fermenter, the explanation would be similar, highlighting its ability to ferment lactose.
* **Option D:** Similarly, if this option represents a known lactose fermenter, it would be incorrect for the same reason.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that the lactose fermentation test is crucial in the identification of enteric pathogens. For example, in the investigation of diarrhea, identifying a pathogen like **Shigella** (lactose nonfermenter) versus **E. coli** (lactose fermenter) can guide treatment.

## Correct Answer: C. Shigella.