**Core Concept**
Spontaneous bacterial peritonitis (SBP) is an infection of the ascitic fluid in patients with cirrhosis, characterized by a polymicrobial infection without any apparent source of infection. The diagnosis of SBP is based on the presence of a high neutrophil count in the ascitic fluid.

**Why the Correct Answer is Right**
The correct answer is a high neutrophil count, specifically ≥ 250 cells/mm³. This is because the presence of neutrophils in the ascitic fluid indicates a bacterial infection. The neutrophils are activated in response to bacterial products, such as lipopolysaccharides, which stimulate the release of pro-inflammatory cytokines. The ascitic fluid analysis is a crucial diagnostic tool for SBP, as it allows for the detection of infection in the absence of overt symptoms.

**Why Each Wrong Option is Incorrect**
* **Option A:**
This option is incorrect because the presence of lymphocytes in the ascitic fluid is not indicative of SBP. Lymphocytes are more commonly associated with viral infections or malignancies.
* **Option B:**
This option is incorrect because the presence of eosinophils in the ascitic fluid is not a reliable indicator of SBP. Eosinophils are more commonly associated with parasitic infections or allergic reactions.
* **Option C:**
This option is incorrect because the presence of monocytes in the ascitic fluid is not a specific indicator of SBP. Monocytes are more commonly associated with chronic infections or inflammatory conditions.

**Clinical Pearl / High-Yield Fact**
A key clinical pearl to remember is that SBP is a medical emergency, and prompt treatment is essential to prevent complications and improve outcomes. The diagnosis of SBP should be suspected in patients with cirrhosis who present with new-onset abdominal pain, fever, or changes in mental status.

**Correct Answer: D. Neutrophil count ≥ 250 cells/mm³.**