Which electrolytes are designated for myocardial rhythm and neuromuscular excitability?

• A. Na, K, Cl
• B. Mg, Ca, K
• C. Zn, Cu, Fe
• D. Mg, Na, PO4

Answer: (B)

The key concept here is how certain cations directly regulate the electrical activity of nerves, muscles, and the heart. Potassium, calcium, and magnesium together are the main ions that finely tune excitability and rhythm in these tissues.

Potassium sets the resting membrane potential and governs repolarization in cardiac and skeletal muscle and in neurons. Small shifts in K+ levels can alter how easily cells fire and how quickly they reset after an impulse, which directly affects rhythm and excitability.

Calcium is crucial for the contraction process in the heart and for neurotransmitter release at neuromuscular junctions. In cardiac cells, Ca2+ entry during depolarization contributes to the plateau phase and the strength of contraction, while in neurons it triggers signaling at synapses, influencing neuromuscular transmission.

Magnesium acts as a stabilizer and modulator of ion channels and enzymes, including the Na+/K+ ATPase. It helps prevent excessive excitability and can influence cardiac rhythm; imbalances can lead to arrhythmias or neuromuscular symptoms.

Together, Mg, Ca, and K cover the essential mechanisms underlying myocardial rhythm and neuromuscular excitability, which is why this combination is the best fit.

The other options either replace a key regulator with ions not primarily focused on excitability in these tissues (such as trace metals) or mix in sodium and phosphate, which have important roles but do not focus as directly on rhythm and neuromuscular excitability as the Mg–Ca–K trio.