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radial tunnel syndrome: A condition in which the radial nerve is compressed at the elbow, causing pain, numbness, tingling, and muscle weakness in the hand and arm.

radiation: Energy in the form of particles or waves, such as x-rays and gamma rays. Radiation is often used to help make a diagnosis, as in x-rays, or as a treatment for cancer.

radiation therapy: Treatment with high-energy rays (from x-rays or other sources) designed to control disease and destroy cancer cells.

radical prostatectomy: Surgery to remove the entire prostate.

radiculopathy: Pain caused by irritation of a nerve as it exits the spinal cord.

radioactive iodine: A radioactive form of iodine that can be used as a tracer during a radioactive iodine uptake test or a radioactive thyroid scan. Much larger amounts are used in the treatment of hyperthyroidism.

radioallergosorbent test: A blood test used for identifying allergens.

radiofrequency: A non-surgical technique using an electromagnetic current to penetrate deep into the body's tissues, which can be used to treat pain, cancer, and heart rhythm disorders, among other conditions.

radiograph: Another name for an x-ray.

radionuclide imaging or scan: Another name for nuclear tests: tests that use tiny amounts of short-lived radioactive substances that can target particular organs or cell receptors to diagnose disease.

radionuclides: Short-lived radioactive chemicals that are used in nuclear imaging tests.

rales: Abnormal lung sounds that might be a sign of fluid buildup congestion in the lungs.

randomized: A feature of many clinical trials in which participants are randomly assigned to either a group to be tested or a control group.

randomized controlled trial: A study in which researchers choose a study population at random; one group receives the intervention (such as a nutritional supplement) and another group receives a placebo. Often referred to as an RCT.

range of motion: The extent of movement—and thus flexibility—in a joint, measured in the degrees of a circle.

rapid eye movement sleep: A period of intense brain activity during sleep, often associated with dreams; named for the rapid eye movements that occur during this time. Also called dreaming sleep.

RAST: Abbreviation for radioallergosorbent test, a blood test used for identifying allergens.

Raynaud's syndrome: A condition in which the blood vessels in the fingers and other extremities narrow in response to cold or stress, causing them to turn white or blue.

RCT: Abbreviation for randomized controlled trial, a study in which researchers choose a study population at random; one group receives the intervention (such as a nutritional supplement) and another group receives a placebo.

RDA: Abbreviation for recommended dietary allowance, the average daily amount of a nutrient that will meet the nutritional needs of almost all (97%–98%) healthy people at specific stages of their lives.

reactive arthritis: Joint problems triggered by bacterial or viral infection elsewhere in the body.

rebound insomnia: Insomnia caused by withdrawal from sleep medication; usually it is at least as serious as before the medication was used.

receptors: Structures on the outside of a cell membrane that permit attachment of specific chemicals.

recessive: A gene that will not be expressed in the offspring unless it is inherited from both the mother and father. A recessive gene from one parent that is paired with a dominant gene from the other parent will be overridden by the dominant gene.

recombinant tissue plasminogen activator: A thrombolytic (clot-dissolving) drug made using recombinant DNA technology; used to dissolve blood clots causing an ischemic stroke, pulmonary embolism, or myocardial infarction.

recommended dietary allowance: The average daily amount of a nutrient that will meet the nutritional needs of almost all (97%–98%) healthy people at specific stages of their lives.

recovery: A process of overcoming addiction to alcohol, other psychoactive substance, or addictive behavior. Often this involves a commitment to abstinence, but sometimes it involves reduced use rather than complete abstinence.

rectocele: A weakening of the vaginal wall that allows the rectum to bulge into the vagina.

rectum: The last 12 centimeters of the colon, through which waste is eliminated from the body.

refraction: The deflection of light as it passes through one medium to another of different density; also refers an eye test to evaluate the eye's ability to focus.

regurgitation: Leakage of blood back into a heart chamber that occurs when a heart valve doesn't close properly

rehabilitative driving specialist: A professional trained to evaluate driving skills and, when appropriate, suggest equipment and adaptations to make driving safer for people with certain physical or mental limitations.

rejection: A reaction that occurs when a person's immune system recognizes a transplanted organ as a threatening substance and tries to rid the body of it.

relapse: The return of symptoms and disease after a person seems to have recovered.

relaxation response: The physical effects of meditation and certain other techniques that are opposite to those of the stress response. Effects include marked drops in oxygen consumption, carbon dioxide expiration, heartbeat, and respiration, as well as stabilization or lowering of blood pressure.

REM: Abbreviation for rapid eye movement, which occurs during a period of sleep characterized by intense brain activity, often associated with dreams.

REM behavior disorder: A sleep disorder marked by movement during REM sleep, when muscles are normally paralyzed.

REM rebound: An increase in REM sleep, often with nightmares, that occurs after deprivation of REM sleep or the withdrawal of REM-suppressing medications.

remission: A lessening in the severity of a disease and its symptoms. In cancer, a reduction in the size of a tumor and its symptoms.

remodeling: Altering a body part. Bone is constantly being remodeled in response to exercise or inactivity. Heart tissue is remodeled in response to a heart attack or high blood pressure.

renal: Pertaining to the kidneys.

renal artery stenosis: Narrowing of an artery that supplies blood to the kidney.

renin: An enzyme released by the kidney that stimulates production of angiotensin and aldosterone, two substances in the body that affect blood pressure.

reperfusion: Re-establishing blood flow, such as when a blockage in a coronary artery that is causing a heart attack is cleared.

reperfusion therapy: Techniques used to restart circulation to part of the heart or brain that has been cut off from blood flow during a heart attack or stroke. Reperfusion may entail clot-dissolving drugs, balloon angioplasty, or surgery.

repetitions: Number of times an exercise calls for a muscle to be worked and released (usually eight to 12). Often referred to as reps.

resection: The surgical removal of a lesion or part or all of an organ or other body structure.

resectoscope: An instrument that permits a surgeon to view the inside of a body cavity in order to remove a part of an organ or structure.

reservoir: The habitat in which an infectious agent normally lives, grows, and multiplies. Reservoirs include human, animal, and environmental reservoirs.

resilience: The ability to adapt to change and recover quickly from setbacks such as illness, injury, or misfortune.

resistance: The ability of a pathogen to withstand drugs previously effective against them. Usually the result of genetic mutation.

resistant hypertension: High blood pressure that does not respond to drug therapy and lifestyle changes.

resorption: The removal of bone tissue, both mineral and protein, by osteoclasts.

respiration: The process by which gases enter the body, including external respiration (breathing), and internal respiration, in which oxygen taken in by the lungs is carried by the blood to tissues and carbon dioxide is removed.

restenosis: Renarrowing of a blood vessel that has been widened with angioplasty.

resting energy expenditure: The rate at which the body burns calories while at rest. Resting energy expenditure accounts for 60%–75% of the daily calories burned.

restless legs syndrome: Achy or unpleasant feelings in the legs associated with a need to move. Most prominent at night, making it hard to fall asleep or stay asleep.

resuscitation: The process of reviving a person who is not breathing or whose heart is not beating using techniques such as artificial respiration and heart massage.

retina: The innermost layer of the eye, which converts light energy to electrical energy and sends visual images to the brain via the optic nerve.

retinal detachment: A condition in which the retina separates from the choroid (the back of the eye) and leads to a loss of vision.

retinoid: A synthetic, vitamin A-like compound.

retrieval: The act of recalling previously learned information, involving the reactivation of particular nerve-cell pathways in the brain related to that piece of information.

retrograde ejaculation: An adverse effect of both prostate surgery and some medications that causes semen to flow back into the bladder rather than out through the penis.

retrospective study: A research method that looks for possible causes for a current disease by examining a study population's past habits.

revascularization: Restoration of blood flow to areas of heart muscle affected by coronary artery disease by means of coronary artery bypass surgery or angioplasty.

reward insufficiency theory: The theory that some people turn to addiction to compensate for an inability to sufficiently experience pleasure.

reward pathway: An interrelated set of brain regions that are all involved in recognizing, experiencing, and remembering pleasurable or rewarding events.

rheumatic disease: Any one of over 100 disorders that cause inflammation in connective tissues.

rheumatism: Pain and stiffness of soft tissues in and around joints.

rheumatoid arthritis: An inflammatory autoimmune disease that typically attacks the connective tissue of the joints, causing them to become painful, inflamed, and sometimes deformed.

rheumatoid factor: An antibody found in about 85% of people with rheumatoid arthritis; also appears in other diseases and sometimes in healthy people.

rheumatologist: A medical doctor trained to diagnose and treat disorders involving inflammation of the joints and other parts of the musculoskeletal system.

rheumatology: The branch of medicine devoted to the study and treatment of diseases marked by inflammation, degeneration, or metabolic problems of the connective tissues (particularly the joints and related structures).

rhinoplasty: Reshaping the cartilage and bone of the nose to achieve the desired profile. Commonly known as a nose job.

rhytidectomy: A surgical procedure that involves removing excess skin and tightening the underlying muscle to correct sagging around the jaws, jowls, and neck. Does not include work on the eyes or forehead. Commonly known as a face lift.

right atrium: The right upper chamber of the heart; it receives partially deoxygenated blood as it returns from the body's tissues and moves it into the right ventricle for distribution to the lungs.

right coronary artery: One of the principal coronary arteries that supplies blood to the heart; this vessel supplies the right and lower part of the heart.

right ventricle: The right lower chamber of the heart; it receives blood from the right atrium for distribution to the lungs.

ringworm: An itchy condition of the scalp caused by a fungal infection. Also known as tinea capitis.

Rinne test: A simple hearing test that uses a tuning fork to determine what type of hearing loss a person has.

risk factor: Any factor that can cause a person to be more likely to develop a disease. For example, smoking is a risk factor for lung cancer.

risky drinking: Drinking that increases the chances of adverse consequences; drinking more than guidelines on moderate drinking suggest.

RLS: Abbreviation for restless legs syndrome, an achy or unpleasant feelings in the legs associated with a need to move. Most prominent at night, making it hard to fall asleep or stay asleep.

rods: Light-sensitive cells in the retina that respond best in darkness and dim light.

root: The portion of the tooth below the gum line.

root canal: A channel in the root of the tooth that contains the pulp.

root canal therapy: A procedure in which diseased pulp tissue is removed from the pulp chamber and root canal and the area is sealed off.

rosacea: A skin condition that enlarges blood vessels in the face, causing redness of the nose and other parts of the face.

rotator cuff: A group of tendons and muscles used to raise the arm from the side and rotating the shoulder.

roughage: Indigestible dietary fiber found in grains, fruits, vegetables, and other foods. Roughage is thought to help prevent conditions such as constipation.

rupture: A tear or break in an organ or tissue. Tissue that protrudes through the rupture is known as a hernia.

ruptured disk: When part of a spinal disk bulges out of a tear or weak spot in the disk's tough outer shell. Also known as prolapsed disk or herniated disk.