The diagnosis remains uncertain because of the mildness of the patient's symptoms, the absence of a family history of the syndrome, and other variables. These borderline conditions are sometimes referred to as marfanoid syndromes.

Marfan syndrome is an autosomal dominant condition caused by a genetic mutation. The mutation occurs on chromosome 15 and affects the gene that encodes a protein called fibrillin-1. Over 100 mutations have been described, all of which impair the function of fibrillin-1.

The precise reasons for the mutations are unknown. How the mutation manifests as the Marfan syndrome is also uncertain. There is mounting evidence that the fibrillin-1 defect allows for unabated activity of transforming growth factor-beta (TGF-beta), which causes the clinical manifestations of the syndrome (eg, hyperextensible joints, arachnodactyly, dislocation of the lens, aortic aneurysm).

Because the condition is inherited in an autosomal dominant pattern, a parent with Marfan syndrome has a 50% chance of passing the defective gene on to his/her offspring.

Some diseases are also associated with features that resemble Marfan syndrome. For example, multiple endocrine neoplasia (MEN) type III is associated with what's been called a marfanoid habitus -- patients commonly have the elongated axial bones and hyperextensible joints seen in true Marfan syndrome. MEN-III is caused by a mutation in the RET gene on chromosome 10. It is inherited in an autosomal dominant pattern.

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