**Core Concept**
Pancreatic islet cell tumors are rare neuroendocrine neoplasms that can produce excessive amounts of various hormones, leading to distinct clinical syndromes. Accurate diagnosis and localization of these tumors are crucial for effective management. The investigation of choice should ideally provide detailed information about the tumor's location, size, and potential spread.

**Why the Correct Answer is Right**
The investigation of choice for pancreatic islet cell tumors is a **Selective Arterial Calcium Injection (SACI) angiography**. This procedure involves injecting calcium into the arteries supplying the pancreas, causing vasoconstriction and highlighting the tumor on angiography. This technique is particularly useful for identifying small tumors and assessing their vascular supply, which is essential for surgical planning. SACI angiography can also help differentiate between pancreatic islet cell tumors and other pancreatic lesions.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan - While CT scans can provide valuable information about the size and location of pancreatic islet cell tumors, they may not be sensitive enough to detect small tumors or those with low calcification.
**Option B:** MRI - MRI can provide detailed images of the pancreas and surrounding structures, but it may not be as effective as SACI angiography in identifying small tumors or assessing their vascular supply.
**Option C:** Endoscopic ultrasound (EUS) - EUS can provide high-resolution images of the pancreas and surrounding structures, but it may not be able to assess the tumor's vascular supply or detect small tumors.

**Clinical Pearl / High-Yield Fact**
SACI angiography is particularly useful for identifying pancreatic islet cell tumors in patients with multiple endocrine neoplasia type 1 (MEN1) syndrome, as these patients are at increased risk of developing these tumors.

**Correct Answer: C. Endoscopic ultrasound (EUS) is not the best answer, since the correct answer is not in the options given. However, I will provide the correct answer as per the given options are not complete.**

However, as the correct answer is not provided among the options, I will provide the correct answer from the given options which is not correct.

If we consider the options given, the best among them would be **Option C. Endoscopic ultrasound (EUS)**, although it's not the ideal investigation for pancreatic islet cell tumors.

If you can provide the complete options, I can give the correct explanation.