athlete’s foot A skin infection caused by a fungus called Trichophyton that thrives within the upper layer of the skin when it is moist, warm, and irritated. The fungus can be found on floors and in socks and clothing, and it can be spread from person to person through contact with these objects. However, without…
atlantoaxial joint The joint between the first (atlas) and second (axis) vertebrae of the neck beneath the skull. The axis features a bony prominence called the odontoid process, about which the atlas rotates. The atlantoaxial joint is a pivot type of joint. It allows the head to turn from side to side. The atlantoaxial joint…
atlas The first vertebra in the neck. It supports the head at the base of the skull. Also known as first cervical vertebra.
atonic Without normal muscle tone or strength. An atonic seizure is one in which the person suddenly loses muscle tone and strength; the person cannot sit or stand upright and, unless supported, falls down.
atopic dermatitis A skin disease characterized by areas of severe itching, redness, scaling, and loss of the surface of the skin. Atopic dermatitis is the most common of the many types of eczema. Atopic dermatitis is frequently associated with other allergic disorders, especially asthma and hay fever. A defect of the immune system within the…
ATP 1 Acute thrombocytopenic purpura. 2 Adenosine triphosphate.
atresia Absence of a normal opening, or failure of a structure to be tubular. Atresia can affect many structures in the body. For example, esophageal atresia is a birth defect in which part of the esophagus is not hollow, and with anal atresia, there is no hole at the bottom end of the intestine.
atria The plural of atrium.
atrial arrhythmia See arrhythmia, atrial.
atrial fibrillation See fibrillation, atrial.
atrial septal defect A hole in the wall (septum) between the upper chambers of the heart (atria). Abbreviated ASD. ASD is a major class of heart malformation. Usually, when clots in veins break off (embolize), they travel first to the right side of the heart and then to the lungs, where they lodge. When there…
atrial septum The wall between the right and left atria of the heart.