**Core Concept**
Sentinel surveillance is a public health strategy that monitors disease patterns through key indicator sites to detect outbreaks early. It focuses on identifying new and existing cases to assess disease spread and inform timely interventions, particularly in infectious diseases.

**Why the Correct Answer is Right**
Sentinel surveillance aims to detect new cases of infection (Option B) and track both old and new cases (Option C) to understand disease dynamics and transmission. Identifying missing cases (Option A) ensures completeness of data and helps detect underreporting. However, "identifying cases free of disability" (Option D) is not a core objective—disability status is not part of disease surveillance metrics. Surveillance focuses on disease presence, not functional status or disability.

**Why Each Wrong Option is Incorrect**
Option A: Identifying missing cases is crucial for detecting underreporting and ensuring data completeness in surveillance.
Option B: Detecting new infections is a primary goal of sentinel surveillance to trigger early public health responses.
Option C: Monitoring both old and new cases helps track disease burden and trends over time.

**Why Option D is Incorrect**
Disability status is not a surveillance metric in sentinel surveillance. The system does not assess or track whether individuals are disabled—only whether they have the disease. This reflects a fundamental mismatch between public health surveillance and clinical disability assessment.

**Clinical Pearl / High-Yield Fact**
Sentinel surveillance focuses on **disease presence**, not functional outcomes. Always remember: **"Surveillance = detection of disease, not disability."**

✓ Correct Answer: D. Identifying cases free of disability