**Core Concept**
Wiskott-Aldrich Syndrome (WAS) is a rare X-linked recessive disorder characterized by immunodeficiency, eczema, thrombocytopenia, and bloody diarrhea. The syndrome results from mutations in the WAS gene, which encodes the WAS protein involved in platelet signaling and lymphocyte activation.

**Why the Correct Answer is Right**
WAS is caused by mutations in the WAS gene, leading to impaired platelet function, reduced platelet count, and increased susceptibility to infections. The WAS protein regulates the actin cytoskeleton in platelets and lymphocytes, affecting their structure and function. The syndrome's clinical features result from the combined effects of impaired platelet aggregation, reduced lymphocyte activation, and increased susceptibility to infections.

**Why Each Wrong Option is Incorrect**
**Option A:** Eczema is a characteristic feature of WAS, often presenting in infancy.
**Option B:** Thrombocytopenia is a hallmark of the syndrome, leading to bleeding complications.
**Option C:** Increased susceptibility to infections, particularly pyogenic infections, is a key feature of WAS.

**Clinical Pearl / High-Yield Fact**
WAS is characterized by a triad of eczema, thrombocytopenia, and bloody diarrhea, which can be remembered using the mnemonic "ETB" - Eczema, Thrombocytopenia, and Bloody diarrhea.

**Correct Answer:** D.