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jaundice Yellow discoloration of the skin caused by the accumulation in the blood of the yellow-brown bile pigment called bilirubin. Jaundice is a primary symptom of many different disorders of the liver and biliary systems.

Bilirubin is formed from hemoglobin as old red blood cells break down. The pigment is absorbed from the blood by the liver, where it is dissolved in water and excreted in bile. The process can be disrupted in one of three ways, causing one of the three types of jaundice—hemolytic, hepatocellular, and obstructive.

Jaundice in a newborn is often caused by hemolytic jaundice, in which the body breaks down too many red blood cells, producing too much bilirubin. This occurs in newborns when the liver has not yet developed the capacity to break down bilirubin, and where bilirubin is concentrated.

In hepatocellular jaundice, the transfer of biliru-bin from liver cells to bile is prevented, causing a buildup of bilirubin. This is usually the result of acute hepatitis or liver failure.

Obstructive jaundice is caused by a block of the bile ducts, which prevents the bile from flowing out of the liver. Obstructive jaundice can also occur if the bile ducts are missing or have been destroyed. As a result, bile cannot pass out of the liver, and bilirubin is forced back into the blood.

JC virus A virus that infects about 65 percent of all children by the age of 14, and that may play a role in the development of medulloblastoma—the most common type of malignant brain tumor found in the young. This type of cancer is diagnosed annually in about one out of every 200,000 children under the age of 15. Very aggressive, this cancer is difficult to treat and is often fatal.

Proteins from the JC virus were found in 20 specimens of brain tumors taken from children, according to researchers at Temple University. The presence of the virus suggests it may play a role in the development of these tumors.

The name of the virus comes from the initials of a patient, John Cunningham, from whom the virus was first isolated in the 1970s. Earlier studies have shown that JC virus causes cancer in rats and mice, and that it can cause brain tumors in certain monkeys. JC virus, a type of neutropic poly-omavirus, typically infects the upper respiratory system in the same way as the common cold. The virus causes no disease unless a patient's immune system is weakened or destroyed. In addition to the brain tumors, the JC virus can cause a fatal brain disease known as progressive multifocal leukoencephalopathy (PML). About four percent of all AIDS patients develop PML.

If scientists can conclusively prove that JC virus does play a role in the brain cancer, it may be possible to develop a vaccine that could help in treating the tumor or preventing its spread.

jealousy See sibling rivalry.

jellyfish stings The jellyfish family includes about 200 species that drift along the shoreline, dragging tentacles capable of stinging when touched. While most stings from jellyfish may cause little harm, some jellyfish can inflict severe stings, causing a child to panic and drown. In the water the shock of the sting often causes the child to jerk away, which only stimulates the tentacles to release more poison. If stung by a jellyfish on dry land, more poison is released if the child tries to rip off the sticky threads of the tentacles.

Symptoms

Stings can cause a severe burning pain with a red welt or row of lesions at the sting site. There also

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