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Definition of A (adenine)
In genetics, A stands for adenine, one member of the A-T (adenine-thymine) base pair in DNA. The other base pair in DNA is G-C (guanine-cytosine).

Each base pair forms a "rung of the DNA ladder." A DNA nucleotide is made of a molecule of sugar, a molecule of phosphoric acid, and a molecule called a base. The bases are the "letters" that spell out the genetic code. In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In DNA base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

Adenine is also one of the bases in RNA. There it always pairs with uracil (U). The base pairs in RNA are therefore A-U and G-C.
Definition of A-
Prefix very much employed in medicine and all of the health sciences, indicating "not, without, -less" as, for example, in alexia (not read), aphagia (not eat), aphonia (not voice, voiceless).

The "a-" usually becomes "an-" before a vowel as, for example, in anemia (without blood), anophthalmia (no eye), anotia (no ear), anoxia (no oxygen).

The prefix "a-" comes from the Greek meaning "not."
Definition of A-T
A progressive neurodegenerative genetic disease characterized by cerebellar ataxia (incoordination and lack of balance), ocular telangiectasia ("red eyes" due to widening of small blood vessels in the conjunctiva), immune defects, and a predisposition to malignancy. Chromosomal breakage is a feature. Ataxia-telangiectasia (A-T) cells are abnormally sensitive to killing by ionizing radiation.

A-T becomes evident in early childhood, usually in the first decade of life. The hallmarks of A-T are lack of balance and slurred speech (due to the ataxia) and telangiectasias (tiny red "spider" veins), which appear in the whites of the eyes or on the surface of the ears and cheeks.

People with A-T are predisposed to leukemia and lymphoma. They are also extremely sensitive to radiation exposure. Most people with A-T have a defective immune system, making them susceptible to recurrent sinus and respiratory infections. Other features of the disease may include diabetes mellitus, premature graying of the hair, difficulty swallowing (which causes choking and drooling), and slowed growth. Children with A-T usually have and maintain normal or above normal intelligence.

There is no cure for A-T and, currently, there is no known therapy to slow the progression of the disease. Treatment is symptomatic and supportive. Physical and occupational therapy may help maintain flexibility. Speech therapy may also be useful. Intravenous immune globulin (IVIG) injections may help supplement the defective immune system. High-dose vitamin regimes have been tried. The prognosis (outlook) for people with A-T is poor. Those with the disease usually die in their teens or early 20s.

A-T is inherited as an autosomal recessive trait with both parents contributing a gene for A-T to the affected child. The gene is on chromosome 11. It is called ATM (which stands for ataxia-telangiectasia mutated). ATM encodes a protein that is predominantly confined to the nucleus of cells and that remains constant throughout all stages of the cell cycle. A disorder called the Nijmegen breakage syndrome (NBS1) is similar to A-T. The ATM and NBS1 genes appear to be in a common signaling pathway that choreographs the cell's responses to genomic damage.
Definition of A. baumannii
Acinetobacter baumannii. See: Acinetobacter
Definition of a.c.
Abbreviation on a prescription meaning before meals; from the Latin "ante cibum", before meals. This is one of a number of hallowed abbreviations of Latin terms that have traditionally been used in writing prescriptions.

Some others:

b.i.d. = twice a day (from "bis in die", twice a day)
gtt. = drops (from "guttae", drops)
p.c. = after meals (from "post cibum", after meals)
p.o. = by mouth, orally (from "per os", by mouth)
p.r.n. = when necessary (from "pro re nata", for an occasion that has arisen, as circumstances require, as needed)
q.d. = once a day (from "quaque die", once a day)
q.i.d. = four times a day (from "quater in die", 4 times a day)
q._h.: If a medicine is to be taken every so-many hours (from "quaque", every and the "h" indicating the number of hours)
q.h. = every hour
q.2h. = every 2 hours
q.3h. = every 3 hours
q.4h. = every 4 hours
t.i.d. = three times a day (from "ter in die", 3 times a day)
ut dict. = as directed (from "ut dictum", as directed)
Definition of A1C
A test that measures how much glucose has been sticking during the past 3–4 months to hemoglobin, the substance in the red blood cells that carries oxygen to the cells of the body. The A1C test is important in diabetes as a long-term measure of control over blood glucose. Even outside of diabetes, an elevated A1C level may be a cardiovascular risk factor.

For more information about A1C, see: Hemoglobin A1C.
Definition of AAA
Abdominal aortic aneurysm: A ballooning or widening of the main artery (the aorta) as it courses down through the abdomen. At the point of the aneurysm, the aneurysm usually measures 3 cm or more in diameter. The aneurysm weakens the wall of the aorta and can end in the aorta rupturing with catastrophic consequences. As the diameter of the aorta increases, the chances of an abdominal aortic aneurysm rupturing rise. A measurement of 5 cm is often used to recommend surgery. Persons with AAA tend to be 60 or over. Men are 5 times more likely than women to have an AAA.
Definition of AAAS
Spoken of as the "triple-AS", the American Association for the Advancement of Science is an organization concerned not only with the biomedical sciences but with all of the sciences. The AAAS publishes the weekly journal "Science", one of the great scientific periodicals. "Science" carries a remarkable range of new scientific information including, for example, findings from the Apollo mission to Mars as well as reports from the project to map the human genome.
Definition of AAD
American Association of Dermatology, one of many important professional societies in the health arena. The AMA (the American Medical Association) is a better known example in the US. Only a small selection of the many health-related organizations is given as a sampler in this DICTIONARY.
Definition of AAFP (American Academy of Family Physicians)
Originally, most physicians in the U.S. (and elsewhere) were family doctors. Then there was a strong move away from family medicine toward the medical specialties in the U.S. The pendulum now has swung back to a more equitable balance between family practice and the medical (and surgical) specialties. The AAFP is a professional society for American family doctors.

The AAFP states that: "The American Academy of Family Physicians is the national association of family doctors. It is one of the largest national medical organizations, with more than 85,000 members in 50 states, D.C., Puerto Rico, the Virgin Islands, and Guam. Until October 3, 1971, it was known as the American Academy of General Practice. The name was changed in order to reflect more accurately the changing nature of primary health care.

"The Academy was founded in 1947 to promote and maintain high quality standards for family doctors who are providing continuing comprehensive health care to the public. Other major purposes of the Academy include:

To provide responsible advocacy for and education of patients and the public in all health-related matters;
To preserve and promote quality cost-effective health care;
To promote the science and art of family medicine and to ensure an optimal supply of well-trained family physicians;
To promote and maintain high standards among physicians who practice family medicine;
To preserve the right of family physicians to engage in medical and surgical procedures for which they are qualified by training and experience;
To provide advocacy, representation and leadership for the specialty of family practice;
To maintain and provide an organization with high standards to fulfill the above purposes and to represent the needs of its members.
"The Academy was instrumental in the establishment of family practice, a derivative of classical general practice, as medicine's twentieth primary specialty. The AMA's Council on Medical Education and the independent American Board of Medical Specialties granted approval to a certifying board in family practice, the basic structural requisite of a medical specialty, on February 8, 1969. Examinations have been given annually since 1970, and recertification examinations annually since 1976.

"The Academy maintains a national headquarters in Kansas City, Missouri. It publishes a clinical journal for physicians in primary care entitled American Family Physician, with a circulation of 156,000; a monthly all-member news and features publication entitled FP Report and a publication on practice management and socioeconomic issues entitled Family Practice Management."
AAMC (Ass Am Medical Colleges)
The American Association of Medical Colleges, a nonprofit association of the 125 accredited U.S. medical schools; the 16 accredited Canadian medical schools; more than 400 major teaching hospitals and health systems; some 90 academic and professional societies representing 75,000 faculty members; and the nation's medical students and residents.

The purpose of the AAMC is to improve health through the advancement of academic medicine. In pursuing this purpose, the AAMC works "to strengthen the quality of medical education and training, to enhance the search for biomedical knowledge, to advance research in health services, and to integrate education into the provisions of effective health care."

The AAMC is responsible for the Medical College Admission Test (MCAT) required of everyone applying to medical school in the U.S. and Canada.
The synbol for the enzyme arylalkylamine N-acetyltransferase and for the gene that encodes it. AANAT is present in animals, bacteria and yeast but in no other living organisms. AANAT belongs to the large superfamily of acetyltransferase enzymes. AANAT is used to make melatonin, a hormone that regulates the body's cycles of sleeping and waking.
Alert and oriented.
2. American Association of Ophthalmology, a professional organization.
3. American Academy of Otolaryngology, a professional organization.
4. American Association of Orthodontists, a professional organization.
5. Amino acid oxidase, an enzyme.
AAOS (American Academy of Orthopaedic Surgeons)
The professional organization of American orthopaedists. Literally, the practice of child straightening, orthopaedics is the branch of surgery that is broadly concerned with the skeletal system (bones).
American Academy of Pediatrics
AAP. Its member pediatricians "dedicate their efforts and resources to the health, safety and well-being of infants, children, adolescents and young adults." According to the Academy, it had as of 1998 some 53,000 members in the United States, Canada and Latin America. Over 34,000 of them were board-certified and called Fellows of the American Academy of Pediatrics (FAAP).

The American Academy of Pediatrics was founded in June, 1930 by 35 pediatricians who met in Detroit in response to the need for an independent pediatric forum to address children's needs. When the Academy was established, the idea that children have special developmental and health needs was a new one. Preventive health practices now associated with child care - such as immunizations and regular health exams - were only just beginning to change the custom of treating children as "miniature adults."

The activities of the Academy now include advocacy for children and youth; public education; research; professional education; membership service and advocacy for pediatricians. One of the Academy's major activities also is to further the professional education of its members; continuing education courses, biannual scientific meetings, seminars, publications and statements from committees and sections form the basis of a continuing postgraduate educational program. More than 30 committees develop many of the Academy's positions and programs. Committees have interests as varied as injury and poison prevention, disabled children, sports medicine, nutrition and child health financing.

The Academy publishes the scientific journal called Pediatrics monthly; Pediatrics in Review, its continuing education journal; and the monthly membership newspaper AAP News. It also periodically publishes manuals on such topics as infectious diseases and school health. In its public education efforts, the AAP produces patient education brochures, Healthy Kids magazine, and a series of child care books written by AAP members. Each year the Academy designates October as Child Health Month to emphasize the importance of preventive health care and other child health issues. The Academy executes original research in social, economic and behavioral areas and promotes funding of research. It maintains a Washington Office to ensure that children's health needs are taken into consideration as legislation and public policy are developed. The AAP's state advocacy staff provides assistance to chapters, promoting issues such as child safety legislation and Medicaid policies that increase access to care for low-income children.

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