**Core Concept:** Nephritic syndrome is a condition characterized by nephrotic syndrome and acute kidney injury, often secondary to an underlying cause like glomerulonephritis. Spontaneous bacterial peritonitis (SBP) is a life-threatening complication of cirrhosis, characterized by bacterial infection of the ascitic fluid.

**Why the Correct Answer is Right:**
In a patient with nephritic syndrome and SBP, the most commonly involved micro-organism is **Streptococcus pneumoniae** (Option A). Streptococcus pneumoniae is a gram-positive coccus that can invade the peritoneum and cause SBP, often due to a compromised immune system in patients with cirrhosis and ascites.

**Why Each Wrong Option is Incorrect:**
Option B (Escherichia coli) is less common in SBP, although it may occur in some cases. Option C (Salmonella) and D (Enterococcus) are less frequently involved in SBP, especially in patients with nephritic syndrome.

**Clinical Pearl:**
Understanding the micro-organisms involved in SBP is crucial to guide the appropriate antibiotic therapy. Combined antibiotic therapy targeting both aerobic and anaerobic bacteria is recommended for treating SBP, as the causative organisms can be both aerobic and anaerobic.

**Correct Answer:** Option A (Streptococcus pneumoniae)