**Core Concept**
The patient is presenting with symptoms suggestive of a Hemolytic Uremic Syndrome (HUS) or a Post-Streptococcal Glomerulonephritis (PSGN), both of which can be triggered by a Streptococcal infection. The key concept here is the diagnosis of a Streptococcal infection.

**Why the Correct Answer is Right**
The correct answer is based on the fact that the patient's symptoms are suggestive of a Streptococcal infection. The presence of fever, icterus, conjunctival suffusion, and haematuria is consistent with a PSGN. The streptococcal infection can be confirmed by a serological test that detects the presence of antibodies against Streptococcal antigens. The most commonly used serological test for this purpose is the Anti-Streptolysin O (ASO) titer test. This test measures the level of antibodies against the Streptolysin O toxin produced by Group A beta-hemolytic Streptococci (GABHS). Elevated ASO titers indicate a recent Streptococcal infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although the Widal test is used to diagnose Typhoid fever, it is not relevant in this case as the patient's symptoms are not suggestive of Typhoid fever.
* **Option B:** The Cold Agglutinins test is used to diagnose autoimmune hemolytic anemia, which is not consistent with the patient's presentation.
* **Option C:** The Coombs test is used to diagnose autoimmune hemolytic anemia or hemolytic disease of the newborn, which is not relevant in this case.
* **Option D:** The ANA test is used to diagnose Systemic Lupus Erythematosus (SLE), which is not consistent with the patient's presentation.

**Clinical Pearl / High-Yield Fact**
The ASO titer test is a useful diagnostic tool in cases of suspected PSGN or HUS, particularly in children. It is essential to note that the ASO titer test may remain elevated for several weeks after the infection, so a positive result should be interpreted in the context of the patient's clinical presentation.

**Correct Answer: D. ANA (Antinuclear Antibody) test.