**Core Concept**
Refeeding syndrome is a potentially fatal condition that occurs when nutrition is reintroduced to a severely malnourished patient too quickly, leading to a rapid shift in electrolytes and fluid balance. This condition is characterized by hypophosphatemia, hypokalemia, and hypomagnesemia, which can lead to cardiac arrest, respiratory failure, and death if not promptly managed.

**Why the Correct Answer is Right**
Refeeding syndrome is primarily monitored for electrolyte imbalances such as hypophosphatemia (B), hypokalemia, and hypomagnesemia (D). Calcium levels (A) are also closely monitored as hypocalcemia can occur due to the shift of calcium into cells. However, ammonia levels (C) are not typically monitored in refeeding syndrome, as they are not directly related to the pathophysiology of the condition. Ammonia levels are more relevant to liver function and encephalopathy.

**Why Each Wrong Option is Incorrect**
**Option A:** Calcium levels are monitored in refeeding syndrome, as hypocalcemia can occur due to the shift of calcium into cells.
**Option B:** Phosphate levels are monitored in refeeding syndrome, as hypophosphatemia is a hallmark of the condition.
**Option D:** Magnesium levels are monitored in refeeding syndrome, as hypomagnesemia can occur due to the rapid shift in electrolytes.

**Clinical Pearl / High-Yield Fact**
Refeeding syndrome is more common in patients who have been malnourished for extended periods, such as those with anorexia nervosa or chronic alcoholism. It is essential to monitor electrolyte levels closely in these patients and gradually reintroduce nutrition to prevent this potentially fatal condition.

✓ Correct Answer: C. Ammonia