**Core Concept**
The patient's presentation of sudden onset diabetes, anorexia, weight loss, and epigastric pain radiating to the back is suggestive of a pancreatic pathology, specifically pancreatic cancer. Pancreatic cancer often leads to diabetes due to the destruction of pancreatic islet cells, which produce insulin.

**Why the Correct Answer is Right**
The correct answer is an investigation that can directly assess the pancreatic gland for any abnormalities, such as tumors or cysts. **Endoscopic Ultrasound (EUS)** is a highly sensitive and specific test for evaluating the pancreas and surrounding structures. It involves inserting a flexible tube with an ultrasound probe into the stomach or duodenum to obtain high-resolution images of the pancreas. EUS can help detect pancreatic tumors, cysts, and other abnormalities that may be causing the patient's symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** CT scan of the abdomen is a useful investigation for assessing the extent of pancreatic cancer and its metastasis but is not the next best investigation for confirming the diagnosis of pancreatic cancer.
**Option B:** MRI of the pancreas is not as sensitive as EUS for detecting small pancreatic tumors or cysts.
**Option C:** ERCP (Endoscopic Retrograde Cholangiopancreatography) is useful for diagnosing and treating conditions like pancreatitis and bile duct obstruction but is not the best initial investigation for pancreatic cancer.

**Clinical Pearl / High-Yield Fact**
When suspecting pancreatic cancer, it's essential to remember that the diagnosis often requires a combination of clinical evaluation, imaging studies, and histopathological examination. The patient's symptoms and laboratory findings should prompt a high index of suspicion for pancreatic cancer, leading to further investigations like EUS.

**Correct Answer:** C.