**Core Concept**
The patient's symptoms, such as fever, icterus (jaundice), conjunctival suffusion (redness of the eyes), and hematuria (blood in the urine), are suggestive of a systemic infection, specifically a condition known as **Weil's disease** or **Leptospirosis**. This disease is caused by the bacterium *Leptospira*, which is typically acquired through contact with contaminated water or soil.

**Why the Correct Answer is Right**
The correct answer is **C. Weil's disease serological test**, which involves detecting antibodies against *Leptospira* in the patient's blood. This test is essential for diagnosing Weil's disease, as it can confirm the presence of the infection. The serological test typically measures the levels of **IgM** and **IgG** antibodies against *Leptospira* in the patient's serum. The presence of IgM antibodies indicates a recent infection, while the presence of IgG antibodies suggests a past infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the diagnosis of Weil's disease. **Rickettsial serological test** is used to diagnose diseases caused by *Rickettsia* bacteria, such as typhus and spotted fever.
* **Option B:** **Malarial serological test** is used to diagnose malaria, which is caused by the *Plasmodium* parasite. While malaria can present with fever and jaundice, the other symptoms in this patient's case are not typical of malaria.
* **Option D:** **Hepatitis serological test** is used to diagnose viral hepatitis, which can cause jaundice and liver dysfunction. However, the other symptoms in this patient's case, such as conjunctival suffusion and hematuria, are not typical of hepatitis.

**Clinical Pearl / High-Yield Fact**
Weil's disease is a zoonotic infection, meaning it can be transmitted from animals to humans. The disease is typically acquired through contact with contaminated water or soil, often in areas with poor sanitation or after swimming in contaminated water. It's essential to consider Weil's disease in patients with symptoms of fever, jaundice, and conjunctival suffusion, especially if they have a history of exposure to contaminated water or soil.

**Correct Answer:** C. Weil's disease serological test.