**Core Concept:** Staging of ovarian cancer is essential for determining treatment options and predicting prognosis. The International Federation of Gynecology and Obstetrics (FIGO) staging system is commonly used for ovarian cancer staging.

**Why the Correct Answer is Right:** In this question, the patient has a unilateral ovarian tumor with ascites positive for malignant cells and positive pelvic lymph nodes. These findings indicate that the disease has spread beyond the ovary, as ascites and positive lymph nodes are both indicative of extra-ovarian involvement. Based on the information provided, the patient has Stage III disease according to the FIGO staging system.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because the presence of ascites and positive pelvic lymph nodes suggest extra-ovarian involvement, which is not present in Stage I disease.

B. This option is incorrect because the presence of ascites and positive pelvic lymph nodes indicate extra-ovarian involvement, which is not present in Stage II disease.

C. This option is incorrect because the presence of ascites and positive pelvic lymph nodes suggest extra-ovarian involvement, which is not present in Stage III disease.

D. This option is incorrect as the presence of ascites and positive pelvic lymph nodes indicate extra-ovarian involvement, which is not present in Stage IV disease.

**Clinical Pearl:** The FIGO staging system for ovarian cancer is essential for deciding treatment options and predicting prognosis. It considers the extent of involvement into four stages: Stage I, II, III, and IV. The described patient has Stage III disease, indicating involvement of pelvic lymph nodes and peritoneal spread but without distant organ involvement (e.g., liver, lungs, or peritoneum). This knowledge is crucial for selecting an appropriate treatment strategy, including surgery, chemotherapy, and radiotherapy.