**Core Concept**
Chlorhexidine (CHX) is a disinfectant and antiseptic agent commonly used in oral care products, surgical scrubs, and skin preparations. The effectiveness of CHX is attributed to its ability to bind to bacterial cell membranes, disrupting the integrity and ultimately leading to cell death.

**Why the Correct Answer is Right**
CHX exerts its antimicrobial action by disrupting the cell membrane of bacteria, fungi, and viruses, thereby inhibiting their replication. However, its effectiveness is limited against certain microorganisms, particularly those with a complex cell wall structure or those that are encapsulated. The correct answer is related to an organism whose cell wall is resistant to CHX's mechanism of action.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to CHX's limitations.
**Option B:** This is not the correct answer, as CHX is effective against most gram-positive bacteria.
**Option C:** This option might seem plausible, but CHX is generally effective against gram-negative bacteria, although its efficacy may be reduced by the presence of an outer membrane.

**Clinical Pearl / High-Yield Fact**
CHX is more effective against gram-positive bacteria due to their simpler cell wall structure compared to gram-negative bacteria. This knowledge is crucial when selecting an antiseptic agent for specific clinical situations.

**Correct Answer:** D.