**Core Concept**
The patient's presentation with severe recurrent peptic ulcer in the proximal jejunum, along with a history of hypercalcemia and a family history of Zollinger-Ellison syndrome, suggests a diagnosis of Zollinger-Ellison syndrome. This condition is characterized by excessive secretion of gastrin, a hormone that stimulates gastric acid secretion.

**Why the Correct Answer is Right**
In Zollinger-Ellison syndrome, a gastrin-secreting tumor (gastrinoma) leads to hypergastrinemia. Elevated gastrin levels stimulate the parietal cells in the stomach to secrete excessive amounts of gastric acid, causing peptic ulcers. To confirm the diagnosis, measuring gastrin levels is essential. A fasting gastrin level is typically elevated in patients with Zollinger-Ellison syndrome, with a level above 200 pg/mL being diagnostic.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastric acid levels - While gastric acid levels are indeed elevated in Zollinger-Ellison syndrome, measuring gastric acid levels is not a direct way to confirm the diagnosis. Gastric acid levels can be elevated in other conditions as well, such as peptic ulcer disease.

**Option B:** Parathyroid hormone (PTH) - PTH is related to the patient's previous hypercalcemia, but it is not directly related to the diagnosis of Zollinger-Ellison syndrome.

**Option C:** Somatostatin - Somatostatin is a hormone that inhibits gastrin secretion, but measuring somatostatin levels is not a diagnostic test for Zollinger-Ellison syndrome.

**Clinical Pearl / High-Yield Fact**
A key feature of Zollinger-Ellison syndrome is the presence of multiple peptic ulcers, often in unusual locations, such as the proximal jejunum. This is due to the excessive gastric acid secretion stimulated by the gastrin-secreting tumor.

**Correct Answer:** . Gastrin.