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Narcissistic personality disorder - a condition in which a person has severely overly inflated feelings of self-worth, grandiosity, and superiority over others. People with narcissistic personality disorder often exploit others who fail to admire them, and are overly sensitive to criticism, judgment, and defeat.

Nasal - relating to the nose.

National Cancer Institute - the U.S. government agency for cancer research and information. The NCI is one of the National Institutes of Health.

Nausea - a feeling or sensation leading to the urge to vomit.

Near point of accommodation - the closest point in front of the eyes at which an object may be clearly focused.

Near point of convergence - the maximum extent the two eyes can be turned inward.

Necrosis - pertaining to the death of tissue.

Needle aspiration - a procedure that uses a thin needle and syringe to collect tissue or drain a lump after using a local anesthetic.

Needle biopsy - use of a needle to extract tissue, cells, or fluid for microscopic examination.

Neoadjuvant therapy - treatment such as chemotherapy, radiation therapy, or hormone therapy which is given before the primary treatment.

Neoplasm - any abnormal growth of new tissue; a proliferation of cells no longer under normal physiologic control. These may be benign (non-cancerous) or malignant (cancerous).

Nephrectomy - surgical removal of the kidney.

Nephritis - inflammation of the kidneys.

Nephrology - the medical specialty concerned with diseases of the kidneys.

Nephropathy - diabetic kidney disease.

Nephrotic syndrome - a condition characterized by high levels of protein in the urine, low levels of protein in the blood, tissue swelling, and high cholesterol.

Nerve conduction tests - procedure to determine nerve impulse generation.

Nerve sparing technique - a surgical technique during a radial prostatectomy in which one or both of the neurovascular bundles controlling erections are spared.

Neural plasticity - ability of the brain and/or certain parts of the nervous system to adapt to new conditions, such as an injury.

Neural prostheses - devices that substitute for an injured or diseased part of the nervous system to enhance the function. Examples include cochlear implants, which can restore hearing to certain hearing-impaired people, and deep brain stimulation, which can improve movement problems in people with Parkinson's disease. 

Neural stimulation - to activate or energize a nerve through an external source.

Neural tube defect - type of birth defect, such as spina bifida, that results from failure of the spinal cord or brain to develop normally in a fetus.

Neuralgia - pain in distribution of nerve or nerves.

Neuritis - inflammation of a nerve or nerves.

Neurofibromatosis - a group of inherited disorders in which noncancerous tumors grow along several nerves; can affect the development of other tissues, including bones and skin, possibly leading to developmental abnormalities.

Neurogenic - of nerve origin.

Neurogenic bladder (also called neuropathic bladder) - a bladder disorder which the bladder cannot be completely emptied of urine because of a problem with the nerves connected to the bladder. It can be caused by a tumor or other condition of the nervous system.

Neurogenic communication disorder - inability to exchange information with others because of hearing, speech, and/or language problems caused by impairment of the nervous system.

Neurological - pertaining to the nervous system.

Neuron - a cell specialized to conduct and generate electrical impulses and to carry information from one part of the brain to another.

Neuropathology - the pathology of the nervous system.

Neuropathy - nerve damage caused by the effects of diabetes.

Neurosonography - the use of ultrasound (high-frequency sound waves) technology to assess neurological conditions.

Neurotransmitters - chemical substances that carry impulses from one nerve cell to another; found in the space (synapse) that separates the transmitting neuron's terminal (axon) from the receiving neuron's terminal (dendrite).

Nigral - of or referring to the substantia nigra.

Nitroglycerin - a medication used to relax or dilate arteries.

Nodule (also called papule) - a solid, raised bump.

Noise-induced hearing loss - hearing loss that is caused either by a one-time or repeated exposure to very loud sound or sounds at various loudness levels over an extended period of time.

Non-Hodgkin lymphoma - a type of lymphoma, a cancer in the lymphatic system; causes the cells in the lymphatic system to abnormally reproduce, eventually causing tumors to grow. Non-Hodgkin lymphoma cells can also spread to other organs.

Noninvasive procedure - a diagnostic effort or treatment that does not require entering the body or puncturing the skin.

Nonsteroidal anti-inflammatory drugs (NSAIDs) - medications that produce fever reducing, analgesic (pain relieving), and anti-inflammatory effects. Examples of NSAIDs include ibuprofen (Motrin, Advil) and naproxen (Aleve).

Nonsyndromic hereditary hearing impairment - hearing loss or deafness that is inherited and is not associated with other inherited clinical characteristics.

Nonulcer dyspepsia - constant pain or discomfort in the upper gastrointestinal (GI) tract that is not caused by an ulcer.

Norepinephrine - a neurotransmitter found mainly in areas of the brain that are involved in governing autonomic nervous system activity, especially blood pressure and heart rate.

Nuclear medicine - a specialized area of radiology that uses very small amounts of radioactive substances to examine organ function and structure.

Nutcracker syndrome - a condition caused by compression of the renal vein between the abdominal aorta and the superior mesenteric artery. This can lead to problems such as blood in the urine. 

Nutrients - proteins, carbohydrates, fats, vitamins, and minerals provided by food and necessary for growth and the maintenance of life.