Webster's New World Medical Dictionary

mitral valve, aorta, skeleton, and skin syn-
drome
See 
MASS syndrome.
mitral valve prolapse
A common heart valve
anomaly affecting 5–10 percent of people that
involves prolapse (flopping backwards) of the
mitral valve leaflets into the left atrium during con-
traction of the heart’s ventricles. This sometimes
allows leakage of blood through the valve opening
(mitral regurgitation). Most people with mitral
valve prolapse have no symptoms, however, those
who do commonly complain of symptoms such as
fatigue, palpitations, chest pain, anxiety, and
migraine headaches. 
mittelschmerz
Pain due to ovulation that usually
occurs at the midpoint between the menstrual peri-
ods. From the German mittel, meaning “middle,”
and schmerz, meaning “pain.”
mixed connective tissue disease
A mixture of
three diseases of connective tissue (the framework
for the cells of the body): systemic lupus erythe-
matosus, scleroderma, and polymyositis. Patients
with mixed connective tissue disease typically have
features of each of these three component diseases.
They also typically have very high blood levels of
antinuclear antibodies (ANAs) and antibodies to
ribonucleoprotein (anti-RNP). The symptoms often
eventually become dominated by features of one of
the three component illnesses, most commonly
scleroderma. The treatment for mixed connective
tissue disease depends on which features are caus-
ing symptoms. Treatment is often directed at sup-
pressing the inflammation in the tissues by using
anti-inflammatory and immunosuppressive medica-
tions. These medications include nonsteroidal 
anti-inflammatory drugs, cortisone drugs/steroids
(such as prednisone), and cytotoxic drugs (such as
methotrexate, azathioprine, and cyclophosphamide).