**Core Concept**
Splenomegaly (enlarged spleen) is a clinical finding often associated with various underlying conditions, including infections, hematological disorders, and liver diseases. The presence of high-grade fever, chills, and rigors in this patient suggests an infectious or inflammatory cause.

**Why the Correct Answer is Right**
The peripheral smear is a diagnostic tool that helps identify abnormal blood cells. In this case, the presence of a firm spleen and right upper quadrant tenderness, combined with high-grade fever and chills, is suggestive of **Infectious Mononucleosis (IMN)**, also known as glandular fever. IMN is caused by the Epstein-Barr Virus (EBV) and is characterized by lymphocytosis (increased lymphocytes in the blood), which can be identified on a peripheral smear. The spleen is often enlarged in IMN due to sequestration of lymphocytes.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while a firm spleen can be present in other conditions like malaria, the clinical presentation of high-grade fever, chills, and rigors, combined with right upper quadrant tenderness, is not typical of malaria.

**Option B:** This option is incorrect because while a firm spleen can be present in other conditions like typhoid fever, the clinical presentation of high-grade fever, chills, and rigors, combined with right upper quadrant tenderness, is not typical of typhoid fever.

**Option C:** This option is incorrect because while a firm spleen can be present in other conditions like lymphoma, the clinical presentation of high-grade fever, chills, and rigors, combined with right upper quadrant tenderness, is not typical of lymphoma.

**Clinical Pearl / High-Yield Fact**
A firm spleen in a young adult with high-grade fever and chills should raise suspicion for Infectious Mononucleosis (IMN). The presence of lymphocytosis on a peripheral smear can help confirm the diagnosis.

**Correct Answer: C. Infectious Mononucleosis**