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macrocytic anemia: The presence of larger-than-normal red blood cells in circulation—even though there are too few of them—caused by lack of folate and vitamin B12.

macronutrients: Substances that provide energy and that the body needs for growth. The main categories are: fat, carbohydrate, and protein.

macrophage: A type of white blood cell that destroys cell debris, bacteria, and foreign agents.

macula: The area in the center of the retina that produces sharp, clear central vision and allows one to see fine detail.

macular degeneration: An eye disease that slowly destroys sharp, clear central vision.

macular edema: The build-up of fluid in the macula caused by fluid leaking from blood vessels in the eye. It can blur and damage vision.

Magenblase syndrome: Swallowing too much air during a meal, causing excessive gas and discomfort. Also known as stomach bubble syndrome.

magnetic resonance imaging: A scan that creates detailed pictures of internal organs; commonly referred to as MRI. Magnetic resonance imaging uses radio waves, a strong magnetic field, and a computer to produce images of organs and internal tissues.

maintenance of wakefulness test: A test to measure sleepiness during the day. A tester measures how long it takes a person who is sitting up in a chair or bed to fall asleep after he or she has been asked to stay awake.

major depression: Depression that interferes with daily life for an extended period. Episodes of major depression during bereavement can be distinguished from normal sadness by such symptoms as persistent feelings of worthlessness, thoughts about death, feelings of guilt, persistent trouble functioning, and marked mental and physical sluggishness.

major histocompatibility complex molecule: Molecules that help protect the body from foreign substances. These molecules display proteins on the surface of cells so that protective immune system cells can kill the protein if it is harmful to the body.

maladaptive stress response: An unhealthy physiological response to stressors, in which the stress response often does not turn off even when the stressor disappears.

malaise: A general feeling of illness that can be a sign of disease.

malignant: Cancerous.

malignant hypertension: A dangerous type of high blood pressure marked by an unusually sudden rise in blood pressure to very high levels, often accompanied by headache, blurred vision, and seizures.

malnutrition: Failure to eat or to properly absorb the nutrients needed for good health.

MAOIs: Abbreviation for monoamine oxidase inhibitors, medications used to treat depression. They work by making the chemical messengers serotonin and norepinephrine more available.

mast cell: A cell involved in allergic reactions. When stimulated, it releases chemicals like histamine that signal infection and cause inflammation.

mastoid bone: The bone in the skull behind the ear.

mastopexy: A cosmetic surgery to reshape and lift drooping breasts. Commonly known as a breast lift.

masturbation: Sexual self-stimulation.

maximizer: A person who typically evaluates all options before making a decision, in an effort to identify the perfect choice, and who never settles for second-best.

meal plan: A detailed guide outlining the amounts and types of food a person should eat each day.

Medicaid: A government program that offers health care for low-income Americans of any age.

Medicare: A government program that offers health care for Americans ages 65 and older.

Medigap insurance: Health insurance policies that fill in the holes in Medicare coverage.

medulla: Refers to the middle of something.

megaloblastic anemia: Fewer than normal healthy red blood cells in circulation, caused by a lack of folate or vitamin B12. Red cells become large and deformed, and are unable to carry oxygen efficiently.

meglitinides: A type of medication taken to treat type 2 diabetes.

melanin: A substance that gives the skin, hair, and eyes their natural color.

melanocytes: Cells located deep in the epidermis that produce melanin, the pigment that colors skin.

melanoma: The most dangerous type of skin cancer.

melatonin: A hormone that regulates the body's sleep-wake cycle.

membrane: A thin layer of tissue that surrounds or lines organs or cavities.

memory T cells and B cells: Immune cells that remember harmful agents that have entered the body in the past. When they spot the substance again, they spur the immune system to eliminate the invader.

Ménière's disease: An illness caused by a fluid imbalance in the inner ear.

meninges: The three membranes that cover and protect the brain and spinal cord.

meningitis: Swelling of the membranes that cover and protect the brain and spinal cord.

menopause: The point marking the end of menstruation, officially designated as one year after a woman's final period.

metabolic equivalents: Units used to estimate the oxygen consumption, or metabolic cost, of physical activity, and, hence, its intensity. One metabolic equivalent (MET) is the estimated energy cost of the body at rest.

metabolic syndrome: A cluster of risk factors that accelerate the progression of heart disease.

metabolism: The chemical reactions that occur in all living organisms to maintain life. An example is converting food into energy that the body needs to function.

metastasis: The spread of cancer cells from the primary site to another part of the body.

metered-dose inhaler: A device that delivers a specific dose of an inhaled asthma medication.

Methanobrevibacter smithii: Bacteria in the gut that help with the digestion of complex sugars.

METs: Abbreviation for metabolic equivalents, units used to estimate the oxygen consumption, or metabolic cost, of physical activity, and, hence, its intensity. One MET is the estimated energy cost of the body at rest.

MHC molecule: Abbreviation for major histocompatibility complex molecule, molecules that help protect the body from foreign substances. These molecules display proteins on the surface of cells so that protective immune system cells can kill the protein if it is harmful to the body.

microalbuminuria: Leakage of small amounts of a protein called albumin into the urine caused by kidney disease or damage.

microaneurysm: A tiny bulge that develops in the wall of a blood vessel.

microbe: A microorganism.

microdermabrasion: A cosmetic procedure in which tiny crystals under high pressure are sprayed on the face. This buffs away the outer layer of skin, eliminates fine lines, and improves the look of the skin.

microgram: A unit of mass, equal to one-thousandth of a milligram. Abbreviated as mcg.

micronized: Reducing a substance to very small particles.

micronutrients: Vitamins and minerals needed to maintain normal body functions and prevent certain illnesses.

microvascular disease: A condition in which the smallest blood vessels in the walls of the heart are narrowed or inelastic.

microvasculature: The body's small blood vessels.

micturition: Emptying the bladder. Also called urination or voiding.

middle ear: The air-filled cavity behind the eardrum that contains the three small bones that transmit sound vibrations to the inner ear.

migraine headache: A severe headache with pain that usually begins on one side of the head. Symptoms may include visual disturbances (called aura), nausea, vomiting, and sensitivity to light or noise.

migraine with aura: A severe headache preceded by visual disturbances such as flashing lights, seeing lines, or a blind spot.

migraine without aura: A severe headache that is not preceded by visual disturbances such as flashing lights. Also known as common migraine.

mild cognitive impairment: The loss of some brain functions, such as memory, thinking, or language, that is noticeable, but doesn't interfere with ability to carry out daily tasks.

milligram: A metric unit of weight equivalent to one-thousandth of a gram. Abbreviated as mg.

mindfulness: A practice with its roots in Buddhism that encourages people to be more fully aware of the present moment. Often achieved through meditation.

mindfulness meditation: A form of meditation with roots in ancient Buddhist practice through which a person has a calm awareness of his or her body and feelings and is fully engaged in the present; also called insight meditation.

mindfulness-based cognitive behavioral therapy: A well-established acceptance-based therapy, used principally in treatment of depression and anxiety.

mindlessness: Acting without full attention to one's surroundings, behavior, or internal experience.

miotic: An eye drop that constricts the pupil; used to treat glaucoma.

mitochondria: Small structures within cells that create break down nutrients and create energy for cells. Known as the power producers or energy factories of cells.

mitral valve: The valve that controls the one-way flow of blood from the left atrium to the left ventricle.

mitral valve prolapse: A valve problem in which one or both of the mitral valve flaps collapse backward into the left atrium. This may allow a small amount of blood to leak backward (regurgitate) through the valve.

mitral valve stenosis: A narrowing of the mitral valve opening that limits blood flow from the left atrium to the left ventricle.

monoamine oxidase inhibitors: Medications used to treat depression. They work by making the chemical messengers serotonin and norepinephrine more available.

monoclonal antibody: An antibody is a protein molecule that can bind to a virus and mark it for destruction by the immune system. A monoclonal antibody is a man-made substance used to treat some viral and other diseases.

monocytes: White blood cells that protect the body from disease by attacking and consuming foreign particles.

monounsaturated fat: A type of fat abundant in vegetable oils such as olive, peanut, sesame, and canola oils. Monounsaturated fats don't raise levels of unhealthy LDL cholesterol, and may even lower it.

Morton's neuroma: A thickening of nerve tissue between the toes that causes irritation, tingling, or a burning pain in the ball of the foot.

motilin: A hormone that helps the small intestine contract and move food through the digestive tract.

motility: The ability of the digestive tract to move its contents.

motor neuron: A nerve cell that directs activity in a specific group of muscle fibers.

motor unit: The pairing of a nerve cell and the group of muscle fibers it commands.

MRI: Abbreviation for magnetic resonance imaging--a scan that creates detailed pictures of internal organs. MRIs use radio waves, a strong magnetic field, and a computer to produce images of organs and internal tissues.

mucosa: Tissue that lines the tube-like structures of the body such as the respiratory and gastrointestinal tracts.

mucous cysts: Small cysts that form between the nail bed and the top joint of the finger.

mucous membrane: A thin layer that lines many cavities and structures in the body that are exposed to air in the environment, such as the nose, mouth, and lungs.

multi-infarct dementia: Memory loss and impaired thinking caused by tiny strokes that are often too small to notice until a sizable area of the brain is affected.

multinodular goiter: An enlargement of the thyroid gland with more than one lump or nodule appearing on the neck.

multiple sclerosis: An autoimmune disease in which the body's immune system mistakenly attacks myelin, the substance that covers nerve fibers.

multiple sleep latency test: A sleep test to measure daytime sleepiness. It times how quickly a person falls asleep when asked to rest in a quiet, darkened room during the day.

murmur: An sound heard during an examination of the heart caused by abnormal flow of blood through heart chambers or valves. Many heart murmurs do not indicate heart problems; others do.

muscle fatigue: Weakness felt in muscles when they have been tired out.

muscle fibers: Cells bundled together to make up muscle tissue. Also known as muscle cells.

muscle-contraction headache: A headache characterized by constant pressure, mild to moderate pain, and the feeling that a tight band is squeezing the head. Also known as a tension headache.

muscular endurance: The ability of muscle to continue to perform without stopping because of fatigue.

muscularis: The thin layer of smooth muscle lining of the colon or rectum.

musculoskeletal: Related to the muscles and the skeleton.

MUSE: Abbreviation for medicated urethral system for erection. In this therapy, a small drug pellet is inserted into the tip of the penis to produce an erection.

mutation: The process by which a change occurs in genetic material and is inherited by the next generation.

myalgia: Pain or tenderness in a muscle.

myasthenia: An abnormal weakness in a muscle or group of muscles.

mydriatic: A type of drug that widens the pupil.

myelin: A fatty material that surrounds and protects some nerve fibers.

myelography: A test that uses a special dye and X-rays to detect spinal cord problems. The dye is injected into the space surrounding the spinal cord, making the spinal cord, spinal canal, and nerve roots appear in detail on the X-ray.

myelopathy: A disorder in which the spinal cord is compressed, diseased, or damaged.

myocardial infarction: Medical term for heart attack, the sudden death of part of the heart muscle from lack of oxygen.

myocardial rupture: Tearing of one of the walls of the heart. It usually occurs immediately after a heart attack.

myocarditis: Inflammation of the heart muscle (myocardium).

myocardium: The middle layer of heart tissue. The muscular myocardium is sandwiched between the outer layer (epicardium) and the inner layer (endocardium).

myocyte: A muscle cell.

myofibrils: Long interlocking strands that make up muscle fibers.

myofilaments: The fundamental muscle proteins that form myofibrils. Myofilaments slide over one another, bunching up and generating force, when a muscle contracts.

myopia: Nearsightedness. An optical error in which light rays meet and focus before reaching the retina, making objects that are far away appear blurry.

myosin: A protein that helps muscle contract and relax.