**Core Concept**
Miner's cramp is a type of occupational cramp that affects individuals who engage in manual labor, particularly those working in mines or other physically demanding environments. It is characterized by sudden, severe muscle contractions that can be debilitating and painful. Miner's cramp is often associated with electrolyte imbalances and fatigue.

**Why the Correct Answer is Right**
Miner's cramp is primarily caused by a deficiency of magnesium ions (Mg2+) in the body. Magnesium is an essential mineral that plays a crucial role in muscle function and relaxation. When magnesium levels are low, muscles may contract abnormally, leading to cramping and spasms. This condition is often seen in individuals who engage in strenuous physical activity without adequate rest or electrolyte replenishment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely to be incorrect because it does not specify a known cause of Miner's cramp. While fatigue is a contributing factor, it is not the primary cause of the condition.
**Option B:** This option may be incorrect because it does not directly address the underlying cause of Miner's cramp. While muscle fatigue is a symptom, it is not the primary reason for the condition.
**Option C:** This option is likely to be incorrect because it does not accurately describe the primary cause of Miner's cramp. While electrolyte imbalances may contribute to the condition, magnesium deficiency is the primary cause.

**Clinical Pearl / High-Yield Fact**
Miner's cramp is often seen in individuals who work in physically demanding jobs without adequate rest or electrolyte replenishment. It is essential to recognize the importance of magnesium in muscle function and relaxation, and to ensure adequate magnesium intake, particularly in individuals with high physical demands.

**Correct Answer: D. Magnesium deficiency**