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Medical Dictionary     X
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  • X
    Symbol for reactance when expressed on ohms.
  • X (in genetics)

    X (in genetics): Although the letter X can be used as a symbol in various ways (such as with X-rays, the X-axis of a graph, etc.), "the X " in genetics and medicine today usually refers to the X chromosome. The X is the sex chromosome that is found twice in chromosomally normal females and singly along with a Y chromosome in chromosomally normal males. The complete chromosome complement consisting of 46 chromosomes including the two sex chromosomes is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males.

  • X chromosome

    X chromosome: The sex chromosome found twice in normal females and singly, along with a Y chromosome, in normal males. The complete chromosome complement (consisting of 46 chromosomes including the two sex chromosomes) is thus conventionally written as 46,XX for chromosomally normal females and 46,XY for chromosomally normal males.

  • X disease
  • X inactivation
  • X ray
    The ionizing electromagnetic radiation emitted from a highly evacuated tube, resulting from the excitation of the inner orbital electrons by the bombardment of the target anode with a stream of electrons from a heated cathode. Ionizing electromagnetic radiation produced by the excitation of the inner orbital electrons of an atom by other processes, such as nuclear delay and its sequelae.

    Alternate names: radiograph

    Alternate names: roentgen ray

  • x wave
    the negative wave in the atrial and venous pulse curves produced when ventricular ejection moves the floors of the atria toward the ventricular apices.
  • X zone
  • X-inactivation center lyonization
    a centrally positioned, unique site of the transcriptionally silenced X chromosome where the process of chromosome inactivation begins and is spread.
  • X-linked

    X-linked: On the X chromosome. "Linked" in genetics does not mean merely associated. An X-linked gene travels with the X chromosome and therefore is part of the X chromosome.

  • X-linked agammaglobulinemia
    an X-linked recessive B-cell immune deficiency condition, with hypo- or agammaglobulinemia; the immune deficiency becomes apparent as maternally transmitted immunoglobulin levels decline in early infancy.

    Alternate names: Bruton agammaglobulinemia

  • X-linked gene
    a gene located on an X chromosome.
  • X-linked hypogammaglobulinemia
    a congenital, primary immunodeficiency characterized by decreased numbers (or absence) of circulating B lymphocytes with corresponding decrease in immunoglobulins of the five classes; associated with marked susceptibility to infection by pyogenic bacteria (notably, pneumococci and Haemophilus influenzae) beginning after loss of maternal antibodies; X-linked recessive inheritance caused by mutation in the Bruton tyrosine kinase gene (BTK) on Xq.
  • X-linked hypogammaglobulinemia with growth hormone deficiency
    hypogammaglobulinemia combined with a reduced number of B cells; characterized by short stature, delayed puberty, and recurrent infections.
  • X-linked ichthyosis
    a form of ichthyosis, with onset at birth or in early infancy and affecting males; characterized by scaling predominantly on the scalp, neck, and trunk that progresses centripetally; the palms and soles are spared; histologic manifestations are hyperkeratosis, a granular layer in the epidermis, and normal epidermal cell turnover; X-linked recessive inheritance, caused by mutation in the steroid sulfatase gene (STS) on Xp.

    Alternate names: steroid sulfatase deficiency

  • X-linked inheritance
    the pattern of inheritance that may result from a mutant gene on an X chromosome.
  • X-linked locus
    any locus that in normal karyotypes is borne on the X chromosome.
  • X-linked lymphoproliferative syndrome
  • X-pattern esotropia
    decreasing convergence from the primary position in both upward and downward gaze.
  • X-pattern exotropia
    increasing divergence from primary position in both upward and downward gaze.
  • x-radiation
  • X-ray

    X-ray: High-energy radiation. It is used in low doses to diagnose diseases and in high doses to treat cancer.

  • x-ray generator
    the electronic device that controls production of x-rays in radiography; a key function is rectification of line voltage to produce a smooth direct current voltage to the x-ray tube.
  • x-ray microscope
    a microscope in which images are obtained by using x-rays as an energy source that are recorded on a very finely grained film; alternately, the image can be enlarged by projection; if film is used, it may be examined with a light microscope at fairly high magnifications.
  • x-ray therapy
    radiation therapy using x-rays; sometimes used ironically to refer to excessive use of diagnostic radiation.
  • X-ray, AP

    X-ray, AP: An X-ray picture in which the beams pass from front-to-back (anteroposterior). An AP film is as opposed to a PA (posteroanterior) film in which the rays pass through the body from back-to-front.

  • X-ray, lateral

    X-ray, lateral: An X-ray picture taken from the side.

  • X-ray, PA

    X-ray, PA: An X-ray picture in which the beams pass from back-to-front (posteroanterior). By contrast an AP (anteroposterior) film is one in which the rays pass through the body from front-to-back.

  • X-strabismus
    strabismus in which exotropia is more marked when one is looking upward or downward than when one is looking straight ahead.
  • Xaa
    Symbol for unspecified amino acid (also expressed as X).
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