What condition results from a deficiency of magnesium?

The condition that results from a deficiency of magnesium is grass tetany. This condition primarily affects grazing animals, especially cattle, and occurs when there is insufficient magnesium in their diet or if they are consuming high-potassium forages, which can interfere with magnesium absorption. Grass tetany is characterized by neurological and muscular symptoms, including muscle tremors, staggering, and in severe cases, seizures or death.

Rickets is related to deficiencies in vitamin D, calcium, or phosphate, and does not directly involve magnesium levels. Copper toxicity pertains to an overload of copper in the body, which is not related to magnesium deficiency. Hypocalcemia refers to low calcium levels in the blood, commonly seen in dairy cows during lactation but is not specifically a result of magnesium deficiency. Therefore, grass tetany is distinctly linked to magnesium deficiency, making it the correct answer.