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Developmental and Hereditary Disorders

A 13-year-old girl has had a gradually worsening ataxia over the past 6 years. She also reports generalized weakness. Examination findings include height and weight to be at the 50th percentile for a 10-year-old, and a peripheral neuropathy. Serum and red-cell lipid profiles are abnormal, suggesting the diagnosis of abetalipoproteinemia. Chylomicrons, very-low-density lipoprotein (VLDL), and low-density lipoprotein (LDL) would be largely absent in the serum as a consequence of a mutation in which gene?

a. Microsomal triglyceride transfer protein

b. Huntingtin

c. Amyloid precursor protein

d. Dystrophin

e. Transfer RNA (tRNA)

The answer is a. This disorder appears to be caused by a mutation in the gene that encodes a subunit of the microsomal triglyceride transfer protein, which results in impaired VLDL formation and consequent decreased vitamin E delivery to the peripheral and central nervous system. In addition to an abnormal plasma lipid profile, patients have disturbed fat absorption. Presumably, it is the disturbed lipid that deforms the erythrocyte cell wall, but erythrocyte production levels are relatively normal. Fat is increased in the liver, and patients exhibit lactose intolerance. CNS amyloid collections develop in Alzheimer disease.

A newborn infant has a cystic swelling at the base of the spine that is covered with hyperpigmented skin and some coarse hair. Which of the following is the most likely explanation?

a. Mongolian spot

b. Spina bifida occulta

c. Nevus flammeus

d. Meningocele

e. Encephalocele

The answer is d. The Mongolian spot is a benign discoloration of the newborn's skin at the base of the spine. It is usually oval, well circumscribed, flat, and slightly hyperpigmented or otherwise discolored. Spina bifida occulta is a defect in the superior elements of the spinal column that is unassociated with meningeal or spinal cord abnormalities. It may be evidenced superficially by a dimple in the skin or a tuft of hair overlying the base of the spine. When there is evagination of the meninges (dura mater and pia arachnoid) about the cord or cauda equina through the defect in the spine, the condition is called a meningocele. Extrusion of meningeal and neural elements together is called a meningomyelocele. An encephalocele is a defect in the skull with extrusion of brain. Nevus flammeus is a congenital port-wine spot, usually developing on the face.

A 26-year-old man diagnosed with von Hippel-Lindau syndrome has a postcontrast computed tomography ...

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