**Core Concept**
Tetany in a patient with a small intestinal fistula and parenteral nutrition can be caused by electrolyte imbalances, particularly hypomagnesemia. Magnesium plays a crucial role in neuronal excitability and muscle contraction. Low magnesium levels can lead to neuromuscular hyperexcitability, manifesting as tetany.

**Why the Correct Answer is Right**
Hypomagnesemia can cause tetany by impairing the release of acetylcholine, a neurotransmitter essential for muscle contraction. Magnesium also stabilizes the neuronal membrane, preventing excessive neuronal firing. In this patient, the low serum magnesium level (1.2 mEq/L) likely contributed to the development of tetany. Additionally, the patient's Crohn disease and parenteral nutrition may have further exacerbated the magnesium deficiency.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperventilation can lead to respiratory alkalosis, which may cause tetany due to the decreased ionized calcium levels. However, the patient's arterial blood gas shows a normal pH and PCO2, making hyperventilation an unlikely cause.

**Option B:** Although the patient's calcium level is low, it is not significantly low enough to cause tetany. The normal pH and lack of other electrolyte imbalances make hypocalcemia an unlikely cause.

**Option D:** Essential fatty acid deficiency can cause a range of symptoms, including skin lesions and fatigue. However, it is not directly related to the development of tetany in this patient.

**Clinical Pearl / High-Yield Fact**
In patients with Crohn disease, parenteral nutrition can lead to electrolyte imbalances, including hypomagnesemia. This is particularly relevant in patients with small intestinal fistulas, as they may have impaired magnesium absorption.

**✓ Correct Answer: C. Hypomagnesemia**