**Core Concept**
Holt-Oram Syndrome (HOS) is a genetic disorder characterized by upper limb abnormalities and congenital heart defects. It is an autosomal dominant condition, meaning a single copy of the mutated gene is sufficient to cause the condition. The syndrome is caused by mutations in the TBX5 gene, which plays a crucial role in the development of the heart and limbs.

**Why the Correct Answer is Right**
Thrombocytopenia, or a low platelet count, is not a feature of Holt-Oram Syndrome. The condition is primarily associated with upper limb abnormalities, such as radial dysplasia or absent radius, and congenital heart defects, including atrial septal defects (ASDs). The TBX5 gene is involved in the development of the heart and limbs, but it does not affect platelet production. Thrombocytopenia is not a characteristic of HOS, and it is not a feature that would be associated with the TBX5 gene mutation.

**Why Each Wrong Option is Incorrect**
**Option A:** Absent radius is actually a feature of Holt-Oram Syndrome. Radial dysplasia or absent radius is a common upper limb abnormality associated with HOS.
**Option B:** Atrial septal defect (ASD) is also a feature of Holt-Oram Syndrome. ASDs are a type of congenital heart defect that can occur in individuals with HOS.
**Option D:** Autosomal dominant inheritance is correct for Holt-Oram Syndrome. The condition is inherited in an autosomal dominant pattern, meaning a single copy of the mutated gene is sufficient to cause the condition.

**Clinical Pearl / High-Yield Fact**
Holt-Oram Syndrome is a classic example of a genetic disorder that affects the development of both the heart and the limbs. It is essential to recognize the characteristic upper limb abnormalities and congenital heart defects associated with HOS to make an accurate diagnosis.

**✓ Correct Answer: C. Thrombocytopenia**