**Core Concept**
The Child-Pugh score is a clinical scoring system used to assess the prognosis of patients with cirrhosis, taking into account five clinical parameters: serum bilirubin, serum albumin, prothrombin time (INR), ascites, and encephalopathy.

**Why the Correct Answer is Right**
To determine the Child-Pugh class, we need to score each parameter and sum them up. The parameters are scored as follows:
- Bilirubin: 0 points for 3 mg/dL
- Albumin: 0 points for > 3.5 g/dL, 1 point for 2.8-3.5 g/dL, and 2 points for < 2.8 g/dL - INR: 0 points for 2.3
- Ascites: 0 points for absent, 1 point for mild, and 2 points for severe
- Encephalopathy: 0 points for none, 1 point for mild, and 2 points for severe

Given the patient's serum bilirubin of 2.5 mg/dL (1 point), serum albumin of 3 g/dL (1 point), INR of 2 (1 point), mild ascites (1 point), and no encephalopathy (0 points), the total score is 4.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is not possible as the patient has mild ascites and no encephalopathy, which would not give a score of 6.
**Option C:** This option is not possible as the patient's INR is 2, which is 1 point, not 2 points.
**Option D:** This option is not possible as the patient's serum bilirubin is 2.5 mg/dL, which is 1 point, not 2 points.

**Clinical Pearl / High-Yield Fact**
The Child-Pugh score is a useful tool for predicting the prognosis of patients with cirrhosis, but it does not take into account other important factors such as the etiology of cirrhosis and the presence of varices.

**Correct Answer: A. Child-Pugh class B.**