**Core Concept**
Mycoplasma is a genus of bacteria characterized by the absence of a cell wall, but they do have a unique feature - a cell membrane that contains sterols, specifically cholesterol. This adaptation allows them to maintain osmotic stability and structural integrity.

**Why the Correct Answer is Right**
Mycoplasma's cell membrane is distinct from other bacteria, which typically have a cell wall composed of peptidoglycan or other polysaccharides. The presence of sterols in Mycoplasma's membrane is essential for maintaining the structural integrity of the cell and preventing osmotic lysis. The unique composition of Mycoplasma's membrane enables them to survive and thrive in environments where other bacteria would be unable to.

**Why Each Wrong Option is Incorrect**
**Option A:** Klebsiella is a genus of Gram-negative bacteria that have a typical bacterial cell wall structure, including a peptidoglycan layer, and do not contain sterols in their membrane.
**Option B:** Staphylococcus is a genus of Gram-positive bacteria that also have a typical bacterial cell wall structure and do not contain sterols in their membrane.
**Option C:** Mycobacteria are a genus of acid-fast bacteria that have a unique cell wall structure, but they do not contain sterols in their membrane.

**Clinical Pearl / High-Yield Fact**
Mycoplasma's unique membrane composition is a key factor in their ability to infect and cause disease in humans, particularly respiratory tract infections such as pneumonia.

**✓ Correct Answer: D. Mycoplasma**