**Question:** Disease under WHO surveillance -

A. COVID-19
B. Dengue Fever
C. Influenza
D. Tuberculosis

**Core Concept:**
The question is testing our understanding of diseases that are monitored by the World Health Organization (WHO) as they pose a global public health concern. These include both infectious and non-infectious diseases that can spread across international borders and require international collaboration for prevention and control.

**Why the Correct Answer is Right:**
The correct answer, D. Tuberculosis, is a notifiable disease under WHO surveillance due to its high incidence, significant morbidity, and mortality worldwide. Mycobacterium tuberculosis, the causative agent, primarily targets the lungs but can also affect other organs. Tuberculosis is treatable and curable with a specific combination of antibiotics.

**Why Each Wrong Option is Incorrect:**
A. COVID-19 (Coronavirus Disease 2019) is caused by SARS-CoV-2 and is under WHO surveillance due to its rapid global spread, high morbidity, and mortality. It is not specifically related to WHO surveillance as a disease.
B. Dengue fever is also under WHO surveillance due to its potential for epidemic outbreaks and its impact on public health. Dengue is caused by dengue viruses and primarily affects humans through mosquito vectors.
C. Influenza (flu) is under WHO surveillance due to its seasonal epidemics, potential pandemics, and the need for international collaboration for vaccine development and distribution. Influenza is caused by viruses and affects the respiratory system.

**Clinical Pearl:**
Understanding diseases under WHO surveillance is crucial for medical professionals as it helps in prioritizing research, treatment, and prevention strategies globally. It also aids in formulating health policies and international cooperation for combating these diseases.