**Core Concept**
Resting membrane potential (RMP) in cardiac muscle is a critical concept in electrophysiology, determined by the balance between potassium (K+) and sodium (Na+) ion movements across the cell membrane. The RMP in cardiac muscle is more negative than in other excitable cells due to a unique ion distribution and membrane properties.

**Why the Correct Answer is Right**
The RMP in cardiac muscle is primarily influenced by the high concentration of potassium ions inside the cell (K+), the low concentration of sodium ions inside the cell (Na+), and the presence of the sodium-potassium pump (Na+/K+-ATPase). The cell membrane is more permeable to potassium ions than to sodium ions, allowing potassium ions to leave the cell, contributing to the negative RMP. The sodium-potassium pump maintains the ion gradient by pumping sodium ions out and potassium ions in, which helps to establish and maintain the RMP.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the primary determinant of the RMP in cardiac muscle.
* **Option B:** This option is incorrect because the RMP in cardiac muscle is not primarily influenced by the concentration of calcium ions.
* **Option C:** This option is incorrect because the RMP in cardiac muscle is not a result of a direct electrical current.

**Clinical Pearl / High-Yield Fact**
The RMP in cardiac muscle is crucial for the proper functioning of the heart, as changes in the RMP can lead to abnormal heart rhythms (arrhythmias). Understanding the factors that influence the RMP is essential for the diagnosis and treatment of cardiac electrophysiological disorders.

**Correct Answer: D. The RMP in cardiac muscle is primarily determined by the high concentration of potassium ions inside the cell and the presence of the sodium-potassium pump.**