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D2 receptor: A type of dopamine receptor (see neurotransmitter receptors) that seems to be particularly important in addiction.

daidzein: A substance found in soybeans.

daily value: A guide to the amount of nutrients in a given food; Daily values are given in percentages based on a 2,000-calorie-per-day diet.

dandruff: A mild and common condition that is characterized by an itchy, flaky scalp and that may extend to the ears, face, and chest. Also known as seborrheic dermatitis.

de Quervain's tendonitis: Painful swelling of the tendons at the wrist that move the thumb.

debility: Weakness or a loss of physical strength.

decibel: A unit of measurement for the loudness of a sound. The highest decibels indicate the loudest sounds.

declarative memory: Memory for facts or events (episodic memory); also called explicit memory.

decoction: An herbal product or tea made by boiling a plant in water.

decongestant: Type of medication used to relieve nasal congestion.

deep sleep: Stage of sleep where the brain is less responsive to outside stimuli.

deep venous thrombosis: A dangerous condition in which blood clots form in veins deep in the body, usually the legs. They may break off and block blood flow in the lungs, seriously damaging organs or causing death.

defenses: Coping strategies a person adopts to make it easier to operate in the world.

defibrillation: The delivery of an electric shock to the heart to stop an abnormal rhythm and restore a normal heartbeat.

defibrillator: A device that delivers an electric shock to the heart to restore normal rhythm. Used to treat cardiac arrest and other dangerous heart rhythm problems.

degenerative disease?: Any disease in which the organs or tissues in the body are damaged progressively over time.

degenerative disk disease: Normal, and sometimes painful, deteriorations in the disks of the spine that occur with age.

degenerative joint disease: Arthritis that occurs when the cartilage in joints breaks down over time; also called osteoarthritis.

degenerative spondylolisthesis: Arthritis of the spine that worsens over time; often caused by aging.

delayed sleep phase syndrome: A pattern of falling asleep and waking up later than wanted that tends to worsen progressively over time.

delirium: Sudden, severe confusion that occurs because of a mental or physical illness.

delta waves: Slow brain activity that occurs when a person is in deep sleep.

delusion: A false or irrational belief held by a person despite evidence to the contrary.

dementia: A loss of brain function that worsens over time and affects memory, thinking, behavior, and language.

dementia pugilistica: Loss of brain function, common among former boxers, caused by repeated blows to the head.

demineralization: The process by which bacteria destroy tooth enamel.

demulcent: A substance that soothes irritated tissues and mucous membranes.

dendrites: The parts of a nerve cell that receive signals from other nerve cells.

dendritic cells: Spidery-looking immune system cells that help protect the body from harmful substances.

denial: A defense mechanism characterized by the inability to recognize or admit that addiction is the cause of problems, rather than a solution or mere byproduct. Denial can also refer to the refusal to accept an upsetting reality, such as a serious illness or death or the feelings that follow either.

dental implant: A metal post inserted into the alveolar bone to support an artificial tooth or other prosthesis.

dentin: The layer of hardened tooth tissue under the enamel and around the pulp.

deoxyribonucleic acid: The substance found in the nucleus of cells that contains the genetic instructions for that living organism.

dermal papilla: A structure situated at the base of the hair follicle that contains nerves and blood vessels that fuel the cellular processes of the developing hair shaft.

dermatomyositis: A rare disease in which the muscles become weak and stiff and a skin rash appears.

dermis: The middle layer of skin that contains most of the skin's structures, like collagen, nerves, glands, and hair follicles.

desiccated thyroid: An extract made of dried animal thyroid glands.

detoxification: The process of removing harmful, or toxic, substances from a person's body.

detrusor instability: The sudden, strong need to urinate due to spasms in bladder muscles. Also called urge incontinence.

detrusor muscle: The layer of muscle in the bladder wall that squeezes urine out of the bladder.

detumescence: The softening of an erection.

diabetes: A disease in which the body does not properly produce or use insulin, resulting in abnormally high levels of sugar (glucose) in the blood.

diabetic ketoacidosis: A complication of diabetes in which substances called ketones build up in the blood to dangerously high levels.

diabetic retinopathy: A complication of diabetes that occurs when the small blood vessels in the retina are damaged. It can impair vision or even lead to blindness.

diaphragm: The dome-shaped sheet of muscle at the base of the lungs that helps move air in and out of the lungs.

diastole: The relaxation phase of the normal heart cycle.

diastolic blood pressure: The bottom number of a blood pressure reading, such as 134/78. It represents the pressure in the arteries when the heart relaxes between beats.

diastolic heart failure: The inability of the heart to relax properly between beats (diastole), making it difficult for the ventricles to fill completely with blood from the atria. This can occur when the heart muscle bulks up due to overwork or other causes or when the heart muscle stiffens and loses it flexibility.

diathermy: Use of high-frequency electric currents to heat deep muscle and joint tissue as a form of physical therapy.

dietary fiber: The part of plant foods the body can't digest or absorb. Also called roughage.

dietary reference intakes: The recommended amount of vitamins and minerals individuals should consume per day.

dietary supplements: Vitamins, minerals, herbs, and other substances taken in hopes of improving health.

digestive tract: The series of hollow organs, including the mouth, esophagus, stomach, small intestine, large intestine (colon), rectum, and anus, which break down and digest food and expel waste.

digit: A finger or toe.

digital rectal examination: An exam in which a doctor inserts a lubricated, gloved finger into the rectum to check for abnormalities.

digitalis: A drug that increases the strength of heart muscle contractions.

dihydrotestosterone: A form of the hormone testosterone that spurs the prostate gland to enlarge (benign prostatic hyperplasia).

dilate: To widen or enlarge.

direct transmission: The immediate transfer of an infectious agent from a reservoir to a susceptible host by direct contact or droplet spread.

discoid lupus erythematosus: A rare form of lupus that causes a rash or scarring of skin.

disk: One of the small, shock-absorbing cushions found between the bones that make up the spinal column (vertebrae).

diskectomy: The surgical removal of part of a disk (a small, shock absorbing cushion located between the bones of the spinal column).

diskitis: Swelling of one or more of the pillow-like disks located between the bones of the spinal column.

dislocation: The movement of a bone from its normal position.

dithiolethione: A substance found in cruciferous vegetables, like cabbage, Brussels sprouts, and broccoli, which may play a role in cancer prevention.

diuretic: A drug that eases the heart's workload and decreases the buildup of fluid in the lungs and other parts of the body by promoting the excretion of water and salts. Diuretics (also called water pills) are used to treat high blood pressure, heart failure, and some congenital heart defects.

diverticulitis: The bulging out of small pouches or sacs of tissue from the colon wall.

diverticulum: A small pouch or sac of tissue that bulges out of the colon wall. The plural form is diverticula.

d-limonene: A substance found in citrus fruits.

DNA: An abbreviation for deoxyribonucleic acid, the substance in cells that contains the genetic instructions that direct their function.

DNR: Commonly used abbreviation for do-not-resuscitate order, a legal document that tells health professionals not to revive a person if his or her heart or breathing stops.

dominant: When the genetic trait carried by one allele of a gene pair eclipses the trait carried by another; for example, when a gene carries alleles for two different eye colors, the dominant eye color gene determines the person's eye color.do-not-resuscitate order: A legal document that tells health professionals not to revive a person if his or her heart or breathing stops. Commonly referred to as a DNR.

dopamine: A chemical messenger (neurotransmitter) that affects movement and thought processes.

Doppler ultrasound: A test that uses sound waves to measure how fast blood is flowing through blood vessels.

dorsal: Near the back of the body or an organ.

dorsal kyphosis: A rounded or curved back caused by spinal fractures from osteoporosis. Commonly called dowager's hump.

double blind: A medical study in which the researchers and participants don't know which group is receiving the medication or treatment being studied and which is getting a placebo (fake, inactive version of the medication).

double-contrast barium enema: An x-ray test done to check for colon cancer or other bowel diseases.

dowager's hump: A rounded or curved back (resembling a hump) caused by spinal fractures from osteoporosis.

doxazosin: Medication used to treat high blood pressure or an enlarged prostate gland.

Dressler's syndrome: Inflammation of the sac surrounding the heart (pericardium).

DRI: Abbreviation for dietary reference intake, the recommended amount of vitamins and minerals individuals should consume per day.

drusen: Tiny yellow deposits in the retina of the eye.

dry eye: Stinging, burning, or irritation that occurs when the eye doesn't produce enough moisture.

dry-powder inhaler: A small device that helps a person breathe in dry medication so it reaches the lungs.

dual energy x-ray absorptiometry: An x-ray test used to measure bone density and check for osteoporosis.

dual-photon absorptiometry: A test to measure bone density, usually in the spine or the hip.

duct: A tube or vessel in the body which carries the secretion of a gland; Secretion examples are tears, breast milk, etc.

duodenitis: Inflammation of the duodenum, which is the upper part of the small intestine.

duodenum: The upper part of the small intestine.

duplex Doppler ultrasound scanning: A tool that uses sound waves to reveal blood flow problems.

Dupuytren's disease: A condition that deforms the hand, causing fingers to curl toward the palm.

dura mater: A thin protective membrane covering the brain and spinal cord.

durable power of attorney: A legal document in which an individual appoints another person to make medical, financial, or other decisions when the individual becomes unable to make those decisions.

dysarthria: A speech disability caused by an injury to the brain centers controlling the face, mouth, neck, or throat. People with dysarthria may be able to understand speech and form the right words in their mind but cannot articulate them.

dyspareunia: Painful sexual intercourse.

dyspepsia: Pain or discomfort in the upper abdomen; upset stomach or indigestion.

dysphagia: Difficulty chewing and swallowing food. Dysphagia is extremely common after a stroke.

dysplasia: Abnormal changes in cells of a tissue. The cells aren't cancerous, but they can sometimes progress to cancer.

dyspnea: Difficulty breathing or shortness of breath.

dystonia: A disorder in which muscles twitch, causing uncontrollable twisting movements.