Dictionary

nuchal Referring to the back of the neck (nape). For example, nuchal rigidity is a stiff neck, sometimes a symptom of meningitis.

nuchal fold scan A prenatal ultrasound test that is done to screen for chromosome disorders such as Down syndrome and Turner syndrome. The scan measures the size of the space behind the neck of the fetus between 10 and 14 weeks of pregnancy and provides an index of the amount of fluid that has accumulated…

nuclear medicine The branch of medicine concerned with the use of radioisotopes in the diagnosis, management, and treatment of disease.

nucleic acid One of the family of large molecules that includes deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleic acids were so named because they were first found in the nucleus of cells, but they have since been discovered to also exist outside the nucleus. See also DNA; RNA.

nucleosome A structure that is responsible in part for the compactness of a chromosome. Each nucleosome consists of a sequence of DNA wrapped around a core of histone, which is a type of protein.

nucleotide A subunit of DNA or RNA that consists of a nitrogenous base (A, G, T, or C in DNA; A, G, U, or C in RNA), a phosphate molecule, and a sugar molecule (deoxyribose in DNA, and ribose in RNA). Thousands of nucleotides are linked to form a DNA or an RNA molecule.

nucleus 1 In cell biology, the structure that houses the chromosomes. 2 In neuroanatomy, a group of nerve cells.

nullipara A woman who has not given birth to a viable child.

nongonococcal urethritis An inflammation of the urethra due to infections other than gonorrhea. Abbreviated NGU. NGU is far more common in men than in women and is transmitted by sexual intercourse. Chlamydia trachomatis is the organism most commonly responsible for NGU, but a number of different organisms can be the cause of NGU.

non-Hodgkin’s lymphoma See lymphoma, non-Hodgkin’s.

nonmelanoma skin cancer Skin cancer that does not involve melanocytes. Basal cell cancer and squamous cell cancer are examples of nonmelanoma skin cancers.

Nonoxynol-9 A potent spermicide (spermkilling agent) used as a contraceptive.