Physical examination of an infant delivered to a 42 year old, gravida 3, para 2, woman, is remarkable for slight hypotonia and a poor Moro reflex. Fuher examination reveals upslanting palpebral fissures, epicanthal folds, excess nuchal skin, an enlarged tongue, clinodactyly of the fifth fingers, and a single transverse palmar crease. Which of the following is the most likely diagnosis?

**Core Concept**
Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21, leading to intellectual disability, characteristic physical features, and increased risk of certain medical conditions. The physical features of Down syndrome are often described as a "mosaic" of various traits, including those mentioned in the question.

**Why the Correct Answer is Right**
The physical examination findings in this infant are highly suggestive of Down syndrome. The upslanting palpebral fissures, epicanthal folds, and excess nuchal skin are classic physical features of the condition. The poor Moro reflex and slight hypotonia are also consistent with Down syndrome. The clinodactyly of the fifth fingers and single transverse palmar crease are additional findings that support this diagnosis. These physical features, in combination with the family history of a 42-year-old mother, increase the likelihood of Down syndrome being the correct diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Edwards syndrome is a different genetic disorder caused by an extra copy of chromosome 18, and its physical features are distinct from those described in the question.

**Option B:** Turner syndrome is a genetic disorder affecting females, characterized by short stature, webbed neck, and failure to develop secondary sexual characteristics, none of which are mentioned in the question.

**Option C:** Klinefelter syndrome is a genetic disorder affecting males, characterized by tall stature, gynecomastia, and infertility, none of which are described in the question.

**Clinical Pearl / High-Yield Fact**
The presence of a single transverse palmar crease is a common finding in Down syndrome and should raise suspicion for this diagnosis, especially in the context of other characteristic physical features.

**Correct Answer:** D. Down syndrome.

✓ Correct Answer: A. Down syndrome