What is decreased in Chvostek sign?

The Chvostek sign is associated with hypocalcemia, which is a condition where there is a low level of calcium in the blood. When evaluating for this sign, if the facial nerve is tapped, a twitching of the facial muscles on the same side of the face occurs. This response indicates increased neuromuscular excitability due to decreased calcium levels.

Calcium plays a critical role in neuromuscular function, and when its levels are low, it can lead to increased irritability and excitability of nerve and muscle cells. Therefore, the presence of a positive Chvostek sign is a clinical indication of hypocalcemia, making it the correct answer in this scenario.

The other options, while they pertain to electrolytes and can affect various bodily functions, are not directly linked to the Chvostek sign. Sodium, potassium, and magnesium have their own roles and effects in the body, but they do not specifically indicate hypocalcemia or manifest through the Chvostek sign.