Which electrolyte is most directly responsible for triggering cardiac muscle contraction by entering the cell and initiating calcium-induced calcium release?

• A. Potassium (K${+}^{}$)
• B. Calcium (Ca$2^{+}$)
• C. Sodium (Na${+}^{}$)
• D. Chloride (Cl${-}^{}$)

Answer: (B)

Verified step by step guidance

1. Understand the role of electrolytes in cardiac muscle contraction, focusing on how ions influence the electrical and mechanical activity of the heart cells.
2. Recognize that cardiac muscle contraction is initiated by an action potential that causes specific ions to move across the cell membrane, triggering further intracellular events.
3. Identify that calcium ions (Ca^{2+}) enter the cardiac muscle cell through voltage-gated calcium channels during the action potential, which is a key step in contraction.
4. Know that the entry of extracellular calcium triggers the release of more calcium from the sarcoplasmic reticulum inside the cell, a process called calcium-induced calcium release, which ultimately leads to muscle contraction.
5. Conclude that among the given electrolytes, calcium (Ca^{2+}) is the most directly responsible for triggering cardiac muscle contraction by entering the cell and initiating this calcium-induced calcium release.