A single cell within a culture of freshly isolated cardiac muscle cells is injected with a fluorescent dye that cannot cross cell membranes. Within minutes, several adjacent cells become fluorescent. The most likely explanation for this observation is the presence of

**Core Concept**
Direct cell-to-cell communication via gap junctions allows for the exchange of ions, metabolites, and signaling molecules between adjacent cardiac muscle cells, enabling coordinated electrical activity and synchronized contraction.

**Why the Correct Answer is Right**
The fluorescent dye cannot cross cell membranes, but the cardiac muscle cells are connected by gap junctions, which are specialized channels that form a direct cytoplasmic bridge between adjacent cells. These gap junctions are composed of connexin proteins, which allow for the exchange of molecules up to 1 kilodalton in size. As a result, the fluorescent dye can diffuse through the gap junctions from the injected cell to adjacent cells, causing them to become fluorescent.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided.
* **Option B:** This option is not provided.
* **Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
Gap junctions play a crucial role in maintaining the synchronized electrical activity of the heart, and their dysfunction has been implicated in various cardiac arrhythmias and conduction disorders.

**Correct Answer: D. Gap junctions**

✓ Correct Answer: A. Gap junctions