**Question:** A young, tall, thin, male with archnodactyly has ectopia lentis in both eyes. The most likely diagnosis is:

A. Marfan syndrome
B. Ehlers-Danlos syndrome
C. Turner syndrome
D. Down syndrome

**Core Concept:**
Archnodactyly is a congenital condition characterized by abnormally shaped fingers and toes, while ectopia lentis refers to the abnormal positioning of the lens in the eye. Both symptoms are commonly associated with Marfan syndrome. Archnodactyly and ectopia lentis are clinical manifestations of Marfan syndrome, which is an autosomal dominant disorder caused by mutations in the FBN1 gene. This gene codes for fibrillin-1, a protein that plays a crucial role in the formation of microfibrils in connective tissues.

**Why the Correct Answer is Right:**
Marfan syndrome is the most likely diagnosis in this case, as it is characterized by archnodactyly and ectopia lentis. The syndrome affects the connective tissues throughout the body, leading to the clinical manifestations mentioned. The FBN1 gene mutation results in fibrillin-1 deficiency, which affects the microfibrils and consequently the development of the skeletal and ocular systems.

**Why Each Wrong Option is Incorrect:**

A. Ehlers-Danlos syndrome is a group of disorders characterized by loose joints, skin laxity, and joint hypermobility. While some patients with Ehlers-Danlos syndrome can present with archnodactyly, it is not as strongly associated with ectopia lentis as Marfan syndrome.
B. Turner syndrome is a genetic disorder typically seen in females, characterized by short stature, webbed neck, and clinodactyly (curved fingers). Ectopia lentis is not a common feature of Turner syndrome.
C. Turner syndrome is a genetic disorder typically seen in females, characterized by short stature, webbed neck, and clinodactyly (curved fingers). Ectopia lentis is not a common feature of Turner syndrome.
D. Down syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of an extra chromosome 21. While some patients with Down syndrome may present with archnodactyly, it is not as strongly associated with ectopia lentis as Marfan syndrome.

**Clinical Pearl:**

Ectopia lentis is a hallmark of Marfan syndrome, as it is a result of abnormal connective tissue development due to fibrillin-1 deficiency. This deficiency leads to the dislocation of the lens from its normal position, causing ectopia lentis. Marfan syndrome is an autosomal dominant disorder, and the gene mutation can affect both genders. Archnodactyly is also a common feature of Marfan syndrome, making it the most likely diagnosis in this case.