Arnold-Chiari malformation type II is a rare neurological disorder caused by a structural defect in the brain. It is characterized by a downward displacement of the cerebellar tonsils through the foramen magnum (the opening at the base of the skul...

Aromatase deficiency is a rare genetic disorder caused by a mutation in the CYP19A1 gene. This gene is responsible for producing the enzyme aromatase, which is responsible for converting androgens (male hormones) into estrogens (female hormones). ...

Aromatase excess syndrome is a rare genetic disorder caused by a mutation in the CYP19A1 gene. This gene encodes the enzyme aromatase, which is responsible for the production of the hormone estrogen. People with this disorder have an excess of est...

Aromatase inhibitors are a class of drugs that inhibit the enzyme aromatase, which is responsible for the conversion of androgens (male hormones) into estrogens (female hormones). By blocking this enzyme, aromatase inhibitors reduce the production...

Aromatic L-Amino Acid Decarboxylase Deficiency (AADC Deficiency) is a rare genetic disorder caused by a deficiency of the enzyme aromatic L-amino acid decarboxylase (AADC). This enzyme is responsible for the conversion of the amino acid L-dopa int...

Arrhinia-choanal atresia-microphthalmia syndrome is a rare genetic disorder characterized by the presence of two or more birth defects. It is characterized by choanal atresia (a blockage of the nasal passage), microphthalmia (abnormally small eyes...

A heart arrhythmia (uh-RITH-me-uh) is an irregular heartbeat. Heart rhythm probl...

A heart arrhythmia (uh-RITH-me-uh) is an irregular heartbeat. Heart rhythm problems (heart arrhythmias) occur when the electrical signals that coordinate the heart's beats don't work properly. The faulty signalling causes the heart to beat too fas...

What is arsenic? Arsenic is a grey-appearing chemical element (atomic number 33, symbol As in the periodic table) also termed a metalloid. Arsenic can exist in a metallic state in three forms (yellow, black, and gray; with gray predominati...

Arterial chemotherapy infusion of the liver and chemoembolization of the liver (transarterial chemoembolization or TACE) are similar procedures that are used for the treatment of cancers in the liver. In both procedures, chemotherapy is injected i...

Arterial dissection-lentiginosis syndrome is a rare genetic disorder characterized by the presence of multiple arterial dissections and lentigines (dark spots on the skin). It is caused by a mutation in the gene encoding the protein elastin, which...

Arterial thoracic outlet syndrome (TOS) is a condition caused by compression of the nerves and/or blood vessels in the thoracic outlet, which is the area between the collarbone and the first rib. Symptoms of TOS can include pain in the neck, shoul...

Arterial Tortuosity Syndrome (ATS) is a rare genetic disorder that affects the arteries of the body. It is characterized by an abnormal twisting and elongation of the arteries, which can lead to an increased risk of aneurysms, strokes, and other v...

Arteriovenous malformation (AVM) is a rare disorder in which abnormal tangles of blood vessels form in the body. These tangles can disrupt the normal flow of blood and can cause a variety of symptoms, including seizures, headaches, and stroke. AVM...

Arteriovenous malformations (AVMs) are defects of the circulatory system that are generally believed to arise during embryonic or fetal development or soon after birth. They are comprised of snarled tangles of arteries and veins. Arteries carry ox...

Arteritis Takayasu is a rare autoimmune disorder that causes inflammation of the aorta and its branches. It is also known as Takayasu arteritis or Takayasu's disease. Symptoms of the condition can include high blood pressure, chest pain, headaches...

Polymyalgia rheumatica is a rare inflammatory disease characterized by muscle pain (myalgia), stiffness, and additional generalized systemic symptoms such as fatigue, low-grade fever, and/or a general feeling of ill health (malaise). Polymyalgia r...