**Core Concept**
Sentinel surveillance is a method of monitoring disease trends or outbreaks by collecting data from a select group of sites or healthcare providers, rather than a comprehensive national or global dataset. This approach is useful for detecting changes in disease incidence or prevalence, particularly for rare or emerging conditions.

**Why the Correct Answer is Right**
Sentinel surveillance relies on a network of healthcare providers or laboratories to report cases of a specific disease or condition. The data collected from these sites is then analyzed to identify trends or patterns. This method is particularly useful for monitoring diseases with a low incidence rate, such as rare infections or vaccine-preventable diseases. The data collected from sentinel surveillance sites can be used to inform public health policy and guide disease control efforts.

**Why Each Wrong Option is Incorrect**
**Option A:** Passive surveillance, which relies on voluntary reporting of cases, is not a type of sentinel surveillance.
**Option B:** Active surveillance, which involves actively searching for cases, is a different approach from sentinel surveillance.
**Option C:** National Notifiable Disease Surveillance System (NNDSS) is a comprehensive national surveillance system, not a type of sentinel surveillance.

**Clinical Pearl / High-Yield Fact**
Sentinel surveillance is a cost-effective and efficient way to monitor disease trends, particularly for rare or emerging conditions. This approach can help identify potential outbreaks or changes in disease incidence, allowing for timely public health interventions.

**Correct Answer: C. National Notifiable Disease Surveillance System is not a type of sentinel surveillance.**