**Core Concept:** Diabetes mellitus can lead to various complications, including diabetic nephropathy, retinopathy, and neuropathy. Cirrhosis in a diabetic patient is a rare complication but can be seen, especially in those with poor glycemic control. Skin hyperpigmentation and restrictive cardiomyopathy are further clues pointing towards this rare complication.

**Why the Correct Answer is Right:** To diagnose this case, we need to identify the cause of cirrhosis and associated complications. A liver biopsy is the gold standard test for diagnosing cirrhosis but is invasive and carries potential risks. Liver function tests (LFT) are non-invasive, easy, and widely available tests that can give us clues about liver disease. In this case, LFT would indicate liver dysfunction, which could be due to diabetes-related complications.

**Why Each Wrong Option is Incorrect:**

A. Serum creatinine and urea levels are important tests in acute kidney injury (AKI) diagnosis, not cirrhosis diagnosis. Therefore, this option is incorrect.

B. Liver ultrasound is a non-invasive test for liver disease, but it doesn't differentiate between various types of liver diseases and doesn't give information about the cause of cirrhosis. Hence, this option is incorrect.

C. HbA1c is a test for glycemic control and not for cirrhosis diagnosis. Given the patient is known diabetic, this option is incorrect.

D. While electrocardiography (ECG) can detect cardiac abnormalities, it doesn't help in diagnosing the cause of skin hyperpigmentation and restrictive cardiomyopathy. Therefore, this option is incorrect.

**Clinical Pearl:** In patients with long-standing poorly controlled diabetes, physicians should be alert for the development of diabetic complications, including liver cirrhosis, hyperpigmentation, and cardiomyopathy. Early detection and management of these complications can improve the patient's prognosis.