L

lactase lactose lactose

An enzyme needed to break down lactose during digestion. One of the sugars found in milk. intolerance The inability to digest lactose due to a deficiency of lactase.

laparoscopy Examination of or surgery in the abdomen using a viewing tube called a 465

laparoscope and special instruments inserted through tiny incisions in the abdomen. G l°ssary large intestine The portion of the digestive tract that extends from the small intestine to the anus. The sections of the large intestine include the cecum, colon, and rectum.

laser skin resurfacing A cosmetic surgery procedure in which a laser is used to treat wrinkles, scars, and various skin lesions.

LDL cholesterol See low-density lipoprotein (LDL) cholesterol.

legionnaires' disease A form of pneumonia caused by a bacterium that contaminates water and air-conditioning systems.

lens The transparent, internal optical component of the eye. lentigines See age spots. LH See luteinizing hormone. libido Sexual desire.

ligament Tough, fibrous tissue that connects bone to bone and provides stability in a joint.

light therapy See phototherapy.

liposuction A cosmetic surgery procedure in which unwanted pockets of fat are removed from the body using a special suction device. Liposuction is the most common type of cosmetic surgery performed on men.

lithotripsy A procedure that uses ultrasonic shock waves to break up stones (such as kidney stones) that have formed in the urinary tract.

liver An abdominal organ that produces chemicals needed by the body and controls the levels of many chemicals in the blood.

liver spots See age spots.

living will An advance directive prepared by a competent person that indicates his or her wishes regarding life-sustaining medical treatments. A living will goes into effect only after the person is unable to speak for himself or herself and can be revised or withdrawn by the person at any time.

Lou Gehrig's disease See amyotrophic lateral sclerosis.

low-density lipoprotein (LDL) cholesterol Also known as "bad" cholesterol. LDL cholesterol is a type of fat carried in the bloodstream that increases the risk of atherosclerosis and coronary artery disease.

466 lumbar puncture A diagnostic procedure in which a hollow needle is used to remove

Glossary cerebrospinal fluid from the lower part of the spinal canal for testing.

luteinizing hormone A hormone produced in the pituitary gland that stimulates secretion of testosterone by the testicles.

Lyme disease A bacterial infection transmitted by a tick bite that causes a rash, fever, and inflammation of the joints and the heart.

lymph nodes Small glands clustered in the neck, armpits, abdomen, and groin that are part of the body's immune system. The lymph nodes supply infection-fighting cells to the bloodstream and filter out bacteria and other antigens.

lymphocyte A specialized white blood cell that has an important role in the immune system. Lymphocytes protect the body from invading microorganisms and cancer cells. Two types of lymphocytes are B lymphocytes (B cells) and T lymphocytes (T cells).

"M macula The part of the retina of the eye that provides sharp sight in the center of the field of vision. The macula is essential for seeing fine detail.

macular degeneration Age-related damage to the macula. Macular degeneration leads to impaired vision.

magnetic resonance imaging A diagnostic technique that uses a computer, a powerful magnetic field, and radio waves to produce detailed two- and three-dimensional images of body tissues that are displayed on a video monitor.

malabsorption disorders Disorders characterized by impaired absorption of nutrients by the small intestine. Examples of malabsorption disorders include celiac disease and lactose intolerance.

male pattern baldness A common inherited form of hair loss in which hair sheds from the top of the head and the temples at the same time. Male pattern baldness can be treated with drugs such as finasteride and minoxidil.

malignant Describes an abnormal growth that is cancerous.

malignant melanoma See melanoma, malignant.

manic-depressive disorder See bipolar disorder.

median nerve A nerve in the arm that controls movement of the forearm and hand and that transmits sensation from part of the hand to the brain.

melanin The pigment that gives skin, hair, and eyes their color.

melanoma, malignant The most serious form of skin cancer, the first sign of which is often a change in an existing mole. Malignant melanoma spreads quickly and can be fatal. The main cause of melanoma is overexposure to sunlight.

Meniere's disease An inner-ear disorder characterized by dizziness, loss of balance, 467

tinnitus, and hearing loss. Gl°ssary meninges The membranes surrounding the brain and spinal cord.

meningioma A rare, slow-growing, benign brain tumor that develops from the meninges.

meningitis Inflammation of the meninges, usually as the result of an infection.

meniscus A crescent-shaped disk of cartilage that reduces friction in joints such as the jaw, wrist, and knee.

metabolism The chemical processes that take place in the body.

metastasis The spread of cancer from its original location to another location in the body.

MI See myocardial infarction.

microsurgery Delicate surgery performed using a special binocular microscope.

migraine A severe, persistent, sometimes disabling headache that occurs on one side of the head and may spread to the other side. A migraine is accompanied by symptoms such as nausea, vomiting, sensitivity to light and noise, fever, chills, aches, and sweating.

ministroke See transient ischemic attack (TIA).

minoxidil An antihypertensive that is also available in nonprescription lotion form to treat male pattern baldness.

mitral incompetence See mitral valve regurgitation. mitral insufficiency See mitral valve regurgitation.

mitral valve The valve in the heart that allows blood to flow from the left atrium to the left ventricle.

mitral valve prolapse A common, usually minor defect of the mitral valve in which the valve does not close properly, allowing small amounts of blood to leak back into the left atrium from the left ventricle.

mitral valve regurgitation Also known as mitral incompetence or mitral insufficiency. Leakage of blood back through the mitral valve into the left atrium each time the left ventricle contracts, leading to increased blood pressure in the blood vessels that carry blood from the lungs to the heart.

mitral valve stenosis A narrowing of the mitral valve opening that increases resistance to blood flow from the left atrium to the left ventricle. Mitral valve stenosis can eventually lead to heart failure.

468 mole A dark-colored growth on the skin that may be flat or raised and that may vary gl°ssary in size. Moles can appear anywhere on the body. In rare cases a mole may develop into a serious form of skin cancer known as malignant melanoma. See melanoma, malignant.

molluscum contagiosum A common, benign, viral infection characterized by tiny lumps on the skin. Molluscum contagiosum is harmless and usually clears up without treatment in a few months. It is often a sexually transmitted disease (STD) in adults.

mononucleosis An infectious disease caused by the herpesvirus Epstein-Barr virus and characterized by fever, sore throat, and swollen glands.

motor neuron disease A group of rare disorders of unknown cause characterized by degeneration of nerves that control muscle activity in the brain and spinal cord, resulting in weakness and wasting of the muscles. See also amyotrophic lateral sclerosis and Parkinson's disease.

MRI See magnetic resonance imaging. MS See multiple sclerosis.

mucous membrane The thin, skinlike lining of the cavities and tubes in the body, such as the digestive tract, urinary tract, and respiratory tract.

mucus A thick, slimy fluid secreted by a mucous membrane to lubricate and protect the part of the body it lines.

multiple sclerosis A progressive, disabling disorder characterized by degeneration of the protective coverings of nerve cells, which interferes with normal functioning of the nervous system.

muscle contraction headache See headache, muscle contraction.

myelography A diagnostic examination of the spinal cord, nerves, and other tissues in and around the spinal canal, in which contrast medium is injected and X rays are taken. The information recorded during the procedure is called a myelogram.

myocardial infarction Also known as a heart attack. Sudden death of a section of the heart muscle because of a loss of blood supply. The most common cause is blockage of blood flow in one of the coronary arteries by a thrombus. Symptoms include severe, constant chest pain; shortness of breath; nausea; vomiting; restlessness; cold, clammy skin; and loss of consciousness. Risk factors include high blood levels of cholesterol, atherosclerosis, obesity, diabetes, and high blood pressure.

myringoplasty A surgical procedure used to repair a hole in the eardrum.

N narcolepsy A sleep disorder characterized by excessive daytime sleepiness, causing sudden, recurrent episodes of sleep throughout the day.

natural crown See crown, natural.

nephron The basic filtering unit of the kidneys. 469

Glossary nephrotic syndrome A collection of symptoms and signs, including high levels of protein in the urine, high levels of cholesterol in the blood, low levels of protein in the blood, and swelling of body tissues, that together indicate kidney damage.

neuron A nerve cell.

neurotransmitters Chemical messengers in the brain.

nongonococcal urethritis Inflammation of the urethra that is not caused by gonorrhea and is the most common type of sexually transmitted disease (STD). It can be caused by infection with Chlamydia trachomatis or a herpesvirus.

nonsteroidal anti-inflammatory drugs Medications such as ibuprofen and naproxen that relieve pain and inflammation.

noradrenaline See norepinephrine.

norepinephrine Also known as noradrenaline. A hormone that helps regulate heart rate and blood pressure by narrowing blood vessels and increasing heart rate when blood pressure drops below the normal level.

NSAIDs See nonsteroidal anti-inflammatory drugs.

oat cell cancer See small cell cancer.

obesity A condition in which a person weighs 20 percent or more over the maximum desirable weight for his or her build and height.

obsessive-compulsive disorder A mental disorder characterized by persistent thoughts or impulses called obsessions that lead to repetitive, ritualized thoughts or behaviors (compulsions).

occlusion See bite.

ophthalmologist A physician who specializes in treating disorders of the eyes.

ophthalmoscopy Examination of the inside of the eye with a handheld, lighted viewing instrument called an ophthalmoscope.

opportunistic infections Infections that rarely occur in healthy people but frequently occur in people who have impaired immune systems—such as people with acquired immunodeficiency syndrome (AIDS).

optic nerve One of a pair of nerves that transmit information about visual images received from the retinas of the eyes to the visual cortex of the brain.

orchiectomy The surgical removal of one or both testicles.

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