**Core Concept**
Seizures or convulsions postoperatively can be caused by various factors, primarily related to the surgical procedure, anesthesia, or underlying medical conditions. The underlying pathophysiology involves an imbalance of neurotransmitters, particularly an excess of excitatory neurotransmitters like glutamate and a relative deficiency of inhibitory neurotransmitters like GABA.

**Why the Correct Answer is Right**
The correct answer is likely **C. Hypocalcemia**. Hypocalcemia occurs when the serum calcium levels drop below the normal range, which can lead to muscle cramps, tetany, and in severe cases, generalized convulsions. The mechanism involves the disruption of calcium homeostasis, which affects neuronal excitability. Calcium plays a crucial role in neurotransmitter release, and a decrease in calcium levels can lead to an overactive neuronal response, resulting in seizures.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the postoperative period or the development of seizures.
**Option B:** While hypoxia can cause seizures, it is not the most direct cause of generalized convulsions 20 minutes postoperatively. Hypoxia typically occurs earlier in the perioperative period.

**Clinical Pearl / High-Yield Fact**
Postoperative seizures can be caused by various factors, including hypocalcemia, hypomagnesemia, and electrolyte imbalances. It is essential to maintain normal electrolyte levels and monitor for postoperative complications.

**Correct Answer:** C. Hypocalcemia