Webster's New World Medical Dictionary

cross-training
Doing two or more aerobic activ-
ities, such as jogging, bicycling, and swimming, on
a regular basis.
crossed embolism
See embolism, paradoxical.
crossing over
Exchanging genetic material
between two paired chromosomes. Crossing over is
a way to recombine the genetic material so that each
person (except for identical twins) is genetically
unique.
crossover study
A type of clinical trial in which
the study participants receive each treatment in a
random order. With this type of study, every patient
serves as his or her own control. Crossover studies
are often used when researchers feel it would be
difficult to recruit participants willing to risk going
without a promising new treatment.
croup
An infection of the larynx, trachea, and
bronchial tubes that occurs mainly in children. It is
usually caused by viruses but sometimes by bacte-
ria. Symptoms include a cough that sounds like a
seal’s bark and a harsh crowing sound during
inhalation. A low-grade fever is common. A major
concern with croup is difficulty breathing as the air
passages narrow. Treatment may include adminis-
tration of moist air (as from a humidifier), saltwa-
ter nose drops, decongestants and cough
suppressants, pain medication, fluids, and, if the
infection is bacterial, antibiotics. The breathing of a
child with croup should be closely monitored, espe-
cially at night, when croup usually gets worse due to
prone body position while sleeping. Although most
children recover from croup without hospitaliza-
tion, some may develop life-threatening breathing
difficulties. Therefore, close contact with a physi-
cian during croup is especially important.
Crouzon syndrome
A hereditary craniofacial
disorder characterized by craniosynostosis, small
eye sockets that cause the eyes to protrude, a large
jaw, and a beaked nose with narrowed breathing
passages. Some people with Crouzon syndrome also
have sleep apnea, hearing loss, and other difficul-
ties. Treatment involves surgery to correct the cran-
iofacial malformations. Also known as craniofacial
dysostosis. See also 
craniosynostosis.