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Homocystinuria is an AR disease characterized by a buildup of homocysteine due to deficiency of cystathionine synthase.
Homocystine = 2 homocysteine molecules bound together by a disulfide bond.
Each homocysteine has an –SH (thiol group), and oxidation makes them join together to form -S–S-.
| The patient in homocystinuria will present as a school-age child with Marfanoid body habitus and thrombotic events. |
The classic USMLE vignette is a 12-year-old girl who is tall and lanky, possibly with scoliosis or kyphosis, who experiences a lens dislocation while at school. They’ll try to trick you into thinking it’s Marfan syndrome, but then they’ll mention something about a thrombotic event or myocardial infarction.
Weird, but homocystinuria is also associated with early-onset osteoporosis.
| If you get a question that sounds like Marfan syndrome in a school-age child, but then they say anything about thrombosis or MI, the answer is homocystinuria. |
Some resources might mention that Marfan syndrome patients have upward dislocation of the lens, whereas in homocystinuria, the lens is dislocated downward + inward, but I can tell you that detail is meaningless for the Step and a question would never hinge on it. I only mention this here because some people might otherwise think that’s an important detail I’ve left out.
If they ask you about which amino acid is essential in homocystinuria, the answer is cysteine because it is deficient. In contrast, methionine is in excess.
| The USMLE likes to assess vitamin B6 as an ancillary Tx for homocystinuria. |
If they ask you about what could help a homocystinuria patient, but cysteine isn’t an answer, look for pyridoxine as an answer choice (vitamin B6), since it is a cofactor for cystathionine synthase.
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