**Core Concept**
Acridine orange is a fluorescent dye that binds to nucleic acids, particularly DNA and RNA, in living cells. This property makes it useful for staining cells and detecting changes in nucleic acid content. The dye's fluorescence is quenched in the monomeric form but is enhanced in the dimeric form, which is more stable in the presence of nucleic acids.

**Why the Correct Answer is Right**
Acridine orange binds to the phosphate backbone of nucleic acids, where it forms a stable complex with the negatively charged phosphate groups. This binding results in the formation of a dimer, which is responsible for the enhancement of fluorescence. The dye's ability to intercalate between nucleotide bases is also thought to contribute to its binding affinity. The fluorescence of acridine orange is further enhanced in the presence of double-stranded DNA, making it a useful tool for detecting DNA content and cell viability.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because acridine orange does not specifically bind to proteins; its binding affinity is limited to nucleic acids.
**Option B:** Incorrect because acridine orange does not have a significant affinity for lipids; its fluorescence is primarily associated with nucleic acid binding.
**Option C:** Incorrect because while acridine orange may interact with cellular membranes, its primary binding site is the nucleic acid phosphate backbone.

**Clinical Pearl / High-Yield Fact**
Acridine orange is a useful tool for detecting changes in nucleic acid content, particularly in cells undergoing apoptosis or necrosis. Its ability to bind to nucleic acids makes it a valuable reagent for staining cells and assessing viability.

**Correct Answer:** A. Nucleic acids.