**Core Concept**
Wiskott-Aldrich syndrome (WAS) is a rare genetic disorder characterized by eczema, thrombocytopenia, immune deficiency, and bloody diarrhea. It is caused by mutations in the WAS gene, which encodes the WAS protein (WASP), a key regulator of the actin cytoskeleton in hematopoietic cells.

**Why the Correct Answer is Right**
The correct answer is not provided, but I will provide explanations for all options. The correct option will be identified later.

**Option A:** WAS is characterized by a decrease in platelet count and size.
**Option A:** This statement is true. Wiskott-Aldrich syndrome is associated with thrombocytopenia (low platelet count) and thrombocytopenic purpura, which is characterized by small platelets.

**Option B:** WAS is associated with a deficiency of T cells and B cells.
**Option B:** This statement is true. Patients with WAS have impaired T cell and B cell function, leading to recurrent infections.

**Option C:** WAS is caused by a mutation in the LYST gene.
**Option C:** This statement is false. Wiskott-Aldrich syndrome is caused by a mutation in the WAS gene, not the LYST gene.

**Option D:** WAS is characterized by the presence of autoimmune antibodies.
**Option D:** This statement is true. Patients with WAS often develop autoimmune antibodies, which contribute to the development of eczema and other autoimmune manifestations.

**Clinical Pearl / High-Yield Fact**
Patients with WAS are at increased risk of developing lymphoma and other hematologic malignancies, likely due to the impaired immune function and increased apoptosis of lymphoid cells.

**Correct Answer:** C. WAS is caused by a mutation in the LYST gene.