Step 1: Understanding the Concept:
Clinical diagnostics rely on serological screening tests to confirm the presence of pathogens inside a patient's body. Many of these tests work on the principle of agglutination (clumping). When a patient is infected with a bacterium, their immune system generates specific antibodies to fight it. In a lab test, mixing the patient's blood serum with known bacterial antigens will cause visible clumping if those specific antibodies are present.
Step 2: Detailed Explanation:
Let's look at the specific diagnostic applications listed in the options:
- (A) Typhoid: Typhoid fever is a severe gastrointestinal bacterial infection caused by Salmonella typhi. The Widal test is a classic serological test designed to detect specific antibodies ($\text{O}$ and $\text{H}$ agglutinins) that the human body develops against this bacterium. If clumping occurs, the test is positive for typhoid fever.
- (B) Malaria: Diagnosed primarily via microscopic blood smear examinations or Rapid Diagnostic Tests (RDTs) tracking protozoan antigens.
- (C) Pneumonia: Diagnosed via sputum cultures, chest X-rays, or blood cell counts.
- (D) Cancer: Screened using biopsies, tumor markers, or advanced imaging techniques (CT, MRI).
Therefore, the Widal test is specifically used for the diagnosis of Typhoid. This matches option (A).
Step 3: Final Answer:
The Widal test is used to diagnose Typhoid.