**Core Concept**
Infectious mononucleosis, also known as glandular fever, is a viral illness caused by Epstein-Barr virus (EBV) characterized by fever, sore throat, and lymphadenopathy. The disease is associated with specific morphological changes in the lymphoid cells, which can aid in diagnosis.

**Why the Correct Answer is Right**
Downey cells, also known as atypical lymphocytes, are a hallmark of infectious mononucleosis. These cells are characterized by their large size, irregular nuclear shape, and prominent nucleoli. The presence of Downey cells in the peripheral blood smear is a diagnostic feature that differentiates infectious mononucleosis from other viral infections. The mechanism behind the appearance of Downey cells is not fully understood, but it is believed to be related to the immune response to EBV, which triggers the proliferation of activated lymphocytes.

**Why Each Wrong Option is Incorrect**
**Option B:** Raquet cells are not associated with infectious mononucleosis; they are actually a type of polymorphonuclear leukocyte (PMN) that can be seen in conditions like chronic myeloid leukemia.
**Option C:** Arbiskov cells are actually associated with chronic granulomatous disease, a genetic disorder that affects the phagocytic cells' ability to kill bacteria.
**Option D:** This option is incorrect because Downey cells are indeed a specific type of cell found in infectious mononucleosis.

**Clinical Pearl / High-Yield Fact**
When examining a patient with suspected infectious mononucleosis, it is essential to look for the presence of atypical lymphocytes in the peripheral blood smear, which can aid in diagnosis. The lymphocytes may also be seen in the cerebrospinal fluid in cases of central nervous system involvement.

**✓ Correct Answer: A. Downey cells.**