**Question:** A fourteen-year-old boy is admitted with a history of fever, icterus, conjunctival suffusion, and haematuria for twenty days. Which of the following serological tests can be of diagnostic utility?

**Core Concept:** Serological tests are laboratory procedures that detect the presence of specific antibodies or antigens in a patient's blood sample. These tests are used to diagnose various infections, diseases, or conditions based on the presence of these biomarkers.

**Why the Correct Answer is Right:** In this scenario, the patient presents with fever, icterus (jaundice), conjunctival suffusion (eye congestion), and haematuria (blood in urine). These symptoms suggest a liver disease or infection that can lead to hemolysis (breakdown of red blood cells), jaundice, and haematuria.

**Why Each Wrong Option is Incorrect:**

A. Serum bilirubin test: This test measures the level of bilirubin in the blood, which is elevated in cases of hemolysis or liver dysfunction. However, this test alone may not provide a definitive diagnosis as it is nonspecific for the underlying cause of hemolysis and liver disease.

B. Liver function tests (LFTs): These tests assess the liver's ability to perform specific functions, such as synthesis and detoxification. While LFTs can provide information about liver damage, they may not directly indicate the cause of hemolysis and jaundice in this case.

C. Coombs' test: Coombs' test is used to detect the presence of antibodies (IgG or IgM) against red blood cells (RBCs) in the blood. It is used to identify autoimmune hemolytic anemia (destruction of RBCs by the patient's immune system). However, this test is not relevant in this scenario since the patient has no history of autoimmune disorders or hemolysis.

D. Direct Coombs' test: Direct Coombs' test detects antibodies directed against RBC membrane components. In autoimmune hemolytic anemia, the presence of IgG antibodies is detected. However, as mentioned earlier, the patient does not have a history of autoimmune disorders or hemolysis.

**Clinical Pearl:** Considering the patient's symptoms and lack of autoimmune history, the most likely cause of hemolysis and jaundice is hemolytic anemia due to malaria infection.

**Why the Correct Answer is Right:**

1. Malaria is a parasitic infection caused by Plasmodium species. It is transmitted through the bite of infected mosquitoes and can lead to hemolysis (destruction of red blood cells) and jaundice (increased bilirubin production) due to liver involvement.

2. In malaria, the presence of Plasmodium antigens (specific malaria proteins) in the blood can trigger an immune response, resulting in the production of antibodies (IgG, IgM) against the parasite. These antibodies can bind to RBCs, causing destruction by the immune system, leading to hemolysis and jaundice.

3. The correct test for malaria infection is a rapid diagnostic test (RDT) for malaria antigens, which would confirm