Where is magnesium primarily stored in the body?

Magnesium is primarily stored in the bones and muscles, making that option the correct choice. In fact, about 50-60% of the body's total magnesium content resides in the skeletal system, which serves as a significant reservoir for this essential mineral. The muscles also contain a substantial amount of magnesium, playing a critical role in muscle function and energy production.

The storage of magnesium in bones is particularly important because it contributes to bone health and structure, while its presence in muscle tissue is vital for muscle contraction, relaxation, and overall neuromuscular function. This intricate distribution underlines magnesium's importance in various physiological processes, including enzymatic reactions, nerve transmission, and regulation of other minerals.

The other options pertain to areas where magnesium is either found in lesser quantities or is not primarily stored. Blood plasma contains magnesium, but the concentration is relatively low compared to the levels found in bones and muscles. The brain and liver do contain magnesium, but they do not serve as the primary storage sites. Cardiac tissue does utilize magnesium for function, specifically in muscle contractions and electrical signaling, but again is not the main storage site for this mineral.