**Question:** A child has been brought to OPD with mental retardation. On examination, microcephaly, triangular face, and fifth finger clinodactyly are found. What is the most probable diagnosis?

**Core Concept:** Microcephaly, triangular face, and fifth finger clinodactyly are clinical features commonly associated with a specific genetic disorder. The correct diagnosis will involve understanding the clinical presentation and known associations with specific genetic syndromes.

**Why the Correct Answer is Right:**

The correct diagnosis is **Cri-du-chat syndrome**. This is a rare genetic disorder resulting from a deletion of part of chromosome 5. The clinical features include:

1. **Microcephaly (small head size)**: Microcephaly is a characteristic feature of Cri-du-chat syndrome, which is caused by the deletion of short arm of chromosome 5.
2. **Triangular face**: The triangular face is another distinctive feature of Cri-du-chat syndrome, reflecting the characteristic facial appearance.
3. **Fifth finger clinodactyly**: This is a rare congenital anomaly where the fifth finger is bent towards the palm, which is seen in Cri-du-chat syndrome due to the deletion of chromosome 5.

**Why Each Wrong Option is Incorrect:**

**Option A:** Down syndrome is another genetic disorder often associated with mental retardation, but it does not involve microcephaly, triangular face, or fifth finger clinodactyly.

**Option B:** Fragile X syndrome is associated with intellectual disability, but it does not typically exhibit microcephaly, triangular face, or fifth finger clinodactyly.

**Option C:** Turner syndrome is a genetic disorder usually involving short stature, webbed neck, and characteristic facial features, but it does not typically present with microcephaly, triangular face, or fifth finger clinodactyly.

**Option D:** Williams syndrome is characterized by intellectual disability, heart defects, and distinctive facial features but does not present with microcephaly, triangular face, or fifth finger clinodactyly.

**Why Each Wrong Option is Incorrect:**

The correct diagnosis of Cri-du-chat syndrome (Option E) is supported by the combination of clinical features observed in the patient, including microcephaly, triangular face, and fifth finger clinodactyly. These features are consistent with the characteristic manifestations of the genetic disorder caused by the deletion of part of chromosome 5.

**Why Each Wrong Option is Incorrect:**

**Option A (Down syndrome):** Down syndrome is caused by trisomy of chromosome 21 and may present with intellectual disability, but not microcephaly, triangular face, or fifth finger clinodactyly.

**Option B (Fragile X syndrome):** Fragile X syndrome is caused by an expansion of the X chromosome and may present with intellectual disability, but it does not typically exhibit microcephaly, triangular face, or fifth finger clinodactyly.

**Option C (Turner syndrome):** Turner syndrome