Medical Dictionary: M

macronutrients: The basic categories of nutrients that humans need: fat, carbohydrate, and protein.

macrophages: White cells, members of the phagocyte family.

macula: The area of the retina packed with cones, responsible for sharp central vision.

magnetic resonance imaging (MRI): A diagnostic technique in which radio waves generated in a strong magnetic field are used to provide information about the tissues within the body; a computer uses this information to produce images of the tissues in many different planes.

major histocompatibility complex (MHC) molecule: There are two classes of MHC molecule on the surface of cells. Every cell in the body has one, but only specific types of antigen-presenting cells have the other. It is the MHC molecules that display the antigens recognized by T cells.

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

malignant: Cancerous.

mast cell: A type of large cell abundant in the mucosa, skin, the lining of the gut, and the airways. Mast cells play a key role in allergic reactions by releasing histamine, leukotrienes, and other inflammatory mediators.

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

mastopexy (breast lift): A procedure to reduce breast droop by removing excess skin and elevating the position of the breasts.

meal plan: A detailed guide outlining the amounts, types, and quantities of food a person should consume daily.

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

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

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

melatonin: A hormone produced in a predictable daily rhythm by the pineal gland.

memory T cells and B cells: Immune cells that remain in the body after initial infection and retain a memory of a pathogen. Memory cells fire up a very fast and powerful immune response when the pathogen appears again.

Ménière’s disease: An illness caused by a fluid imbalance in the inner ear. Symptoms include dizziness, tinnitus, and periodic hearing loss.

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

menopause: The point marking the end of menstruation and childbearing; defined by the World Health Organization as one year after the last period.

metabolism: The rate at which your body uses energy, measured in calories.

metastases: Secondary cancers that form after cancer cells spread to other parts of the body.

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

metered-dose inhaler: A device for delivering inhaled asthma medications. Multiple doses of medication are held within a pressurized canister. With each use, the exact “metered” dose is released in the form of a spray.

microalbuminuria: Presence of small amounts of albumin in the urine; indicates early-stage kidney damage.

microdermabrasion: A procedure to reduce fine lines and wrinkles and to even skin tone by applying aluminum hydroxide crystals to the skin under high pressure.

micronutrients: The vitamins and minerals that humans need to maintain normal body functions and prevent certain illnesses.

micturition: Emptying the bladder; another name for urination or voiding.

middle ear: The space between the outer ear and the inner ear that contains the three ossicles, or bones, involved in hearing.

migraine headache: A headache, usually occurring only on one side of the head, sometimes preceded by visual disturbances and often accompanied by nausea, vomiting, or sensitivity to light or noise.

migraine with aura: A migraine headache preceded by neurological symptoms that may include visual disturbances such as scintillations — sensation of flashing lights or lines — or a blind spot; also known as classic migraine.

migraine without aura: A migraine headache not preceded by neurological symptoms; also known as common migraine.

mild cognitive impairment: A condition characterized by the loss of a single cognitive function — usually memory — that is more severe than is normal for a person’s age, but without obvious impairment in activities of daily living. It is a risk factor for Alzheimer’s disease.

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

mindfulness: Insight or mindfulness meditation is an ancient Buddhist practice that induces the relaxation response by focusing you on the distracting thoughts and sensations that may occur during meditation. Also refers to non-meditative techniques that encourage you to slow your pace and live fully in the moment.

miotic: A type of eye drop that constricts the pupil; used to treat glaucoma.

mitochondria: Often called the body’s energy factories, these small, threadlike structures within the cell’s cytoplasm are where ATP, the energy molecule, is made.

monoamine oxidase inhibitors (MAOIs): Antidepressant medications that act by preventing the breakdown of the monoamines serotonin and norepinephrine.

monounsaturated fat: A type of fat found in vegetable oils such as olive and canola. It has a healthful effect on blood cholesterol levels. They are liquid at room temperature. They also contain one double bond between adjacent carbon atoms.

Morton’s neuroma: A painful inflammation of a nerve in the foot that develops after compression, injury, or mechanical irritation.

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

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

mucous cysts: small cysts that form between the nail bed and the top joint of the finger, most commonly in people with osteoarthritis.

multinodular goiter: A goiter that contains more than one lump or nodule. This type of goiter can evolve into a toxic nodular goiter and cause hyperthyroidism if the nodules are actively producing thyroid hormone.

muscularis: The muscular lining of the colon or rectum.

MUSE: Medicated urethral system for erection, a therapy that involves using a device to insert a drug pellet into the tip of the penis. The drug produces an erection by widening the blood vessels that lead to the corpora cavernosa.

mydriatic: A type of drug that dilates the pupil.

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

myelography: A diagnostic technique in which x-rays are taken of the spine after a contrast medium has been injected into the space within the sheath that surrounds the spinal cord and the cauda equina. This test enables a radiologist to see distortions in the shape of the space, such as those produced by a herniated disk, tumor, fracture, or spinal stenosis.

myelopathy: Compression of the spinal cord.

myocardial infarction: The sudden death of part of the heart muscle from lack of oxygen. Also known as a heart attack.

myopia: An optical error in which light rays meet and focus before reaching the retina; also known as nearsightedness.