**Core Concept:** Functional Endoscopic Sinus Surgery (FESS) is a minimally invasive surgical technique used to treat chronic rhinosinusitis, a condition characterized by inflammation and swelling of the lining of the nasal and paranasal sinuses. The procedure aims to restore proper sinus drainage and relieve symptoms such as facial pain, nasal congestion, and reduced sense of smell.

**Why the Correct Answer is Right:** The correct answer, D, refers to the **diagnostic phase** of FESS. In this phase, a thorough evaluation is performed to determine the extent and severity of the sinus disease and identify the specific anatomic areas involved. This involves visualizing the nasal and sinus cavities using a flexible endoscope, evaluating the mucosal changes, and assessing the patency of the ostia (air passages leading to the sinuses). This information helps guide the surgical plan and ensures a tailored approach for the patient's condition.

**Why Each Wrong Option is Incorrect:**

A. **Preoperative planning:** While essential, this step comes after the diagnostic phase and involves determining the surgical technique, instruments, and plan for the procedure.

B. **Medical management:** FESS is a surgical intervention and not a therapeutic measure for treating sinusitis. Medical management precedes the surgical procedure.

C. **Operative phase:** This refers to the actual surgical procedure and is not a step that occurs before the diagnostic phase.

**Clinical Pearl:**

Understanding the sequential steps in FESS helps ensure a successful and personalized surgical intervention for patients suffering from chronic sinusitis. A thorough diagnostic phase enables optimal surgical planning and increases the likelihood of resolving the patient's symptoms.

**Correct Answer:** D (Preoperative Planning)