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This 7-year-old boy who was said to have decreased vision in both eyes developed severe pain in the left eye, prompting the family to bring him to an eye doctor.  The family claims that the child is otherwise healthy.  The visual acuity is 20/65 in the right eye and 20/300 in the left eye with very high myopic correction.  Pictures of the anterior segment of the left eye are shown. The lens in the right eye is also observed to be subluxated infero-nasally.  This boy is slightly taller than average, has blond hair and is said to have a behavior problem in school.

1.	What is the most likely diagnosis:
	c -- This patient has a dislocated lens in the anterior chamber in the left eye. Infero-nasal subluxation of the fellow eye is relatively a typical finding for homocystinuria as is the age, hair color, and behavior.  Lens subluxation in Marfan's syndrome tends to occur later in life.  Patients with tall stature and lens subluxation suggest a diagnosis of either Marfan or homocystinuria, although there are exceptions. Marfan patients tend to be very tall with arachnodactyly. Patients with homocystinuria tend to be tall but not as tall as Marfan's patients and have less obvious skeletal abnormalities.  The patient has an iris so aniridia is excluded.
2.	Which test would be most helpful in confirming the diagnosis:
	b -- The urine test for sodium-nitroprusside will be helpful to differentiate specially between homocystinuria and Marfan's syndrome. This test is necessary because in some cases these two conditions can be clinically similar.
3.	What is the next best step for this patient:
	c -- The mechanism of the subluxation is defective zonules.  A patient with a freely moving lens inside the eye and intermittent severe eye pain suggests the patient is having intermittent pupillary block. Although after dilating the pupil the lens can drop back behind the iris, the best way to avoid further problems is lensectomy. Until the time of surgery the patient should be placed on long acting dilating drops, like atropine, to avoid pupillary block, or the patient can receive miotic therapy once the lens is safely in the posterior chamber.  Before instituting a general anesthetic the patient should be given a therapeutic dose of vitamin B6 to avoid a thromboembolic event.