What is True is Klinefelter's syndrome - and explain in detail?
Klinefelter Syndrome 

Klinefelter syndrome is the most common chromosomal disorder associated with male hypogonadism and infertility.
It is defined classically by a 47, XXY karyotype with variants demonstrating additional X and Y chromosomes.

Pathophysiology :

The addition of more than 1 extra X or Y chromosome to a male karyotype results in variable physical and cognitive abnormalities.
In general, the extent of phenotypic abnormalities, including mental retardation, is related directly to the number of supernumerary X chromosomes.
As the number of X chromosomes increases, somatic and cognitive development are more likely to be affected.
Gonadal development is particularly susceptible to each additional X chromosome, resulting in seminiferous tubule dysgenesis and infertility as well as hypoplastic and malformed genitalia.
Moreover, mental capacity diminishes with additional X chromosomes. The intelligence quotient (IQ) is reduced by approximately 15 points for each supernumerary X chromosome.
All major areas of development, including expressive and receptive language and coordination, are affected by extra X chromosome material.

Sex :

Because of an additional X chromosome on an XY background, this condition is seen in males only.

Age :

Most males born with Klinefelter syndrome go through life without being diagnosed. Diagnosis, when made, usually occurs in adulthood. The most common indications for karyotyping are hypogonadism and infertility. Physical Growth
Infants and children have normal heights, weights, and head circumferences. About 25% have clinodactyly. Height velocity is increased by age 5 years, and adult height usually is taller than average. Affected individuals also have disproportionately long arms and legs.
Some individuals with Klinefelter variant 49, XXXXY have short stature.
	Central nervous system
Most 47, XXY males have normal intelligence. Family background influences IQ. Subnormal intelligence or mental retardation may be associated with the presence of a higher number of X chromosomes.
About 70% of patients have minor developmental and learning disabilities. These may include academic difficulties, delayed speech and language acquisition, diminished short-term memory, decreased data retrieval skills, reading difficulties, dyslexia, and attention deficit disorder.
Patients may exhibit behavioural problems and psychological distress. This may be due to poor self-esteem and psychosocial development or a decreased ability to deal with stress.
Psychiatric disorders involving anxiety, depression, neurosis, and psychosis are seen more commonly in this group than in the general population.

Sexual characteristics

Patients may lack secondary sexual characteristics because of a decrease in androgen production. This results in the sparse facial/body/sexual hair, a high-pitched voice. They have eunuchoid body habitus.
By late puberty, 30-50% of boys with Klinefelter syndrome manifest gynecomastia, which is secondary to elevated estradiol levels and increased estradiol/testosterone ratio. The risk of developing breast carcinoma is at least 20 times higher than normal.
Testicular dysgenesis (small firm testis, testis size <10 mL) may be present in postpubertal patients.
Infertility/azoospermia may result from atrophy of the seminiferous tubules. Infertility is seen in practically all individuals with a 47, XXY karyotype. Patients with Klinefelter syndrome mosaicism (46, XY/47, XXY) can be fertile.
Patients may have an increased frequency of extragonadal germ cell tumors such as embryonal carcinoma, teratoma, and primary mediastinal germ cell tumor. Cardiac and circulatory problems
Mitral valve prolapse occurs in 55% of patients.
Varicose veins occur in 20-40% of patients.
The prevalence of venous ulcers is 10-20 times higher than normal, and the risk of deep vein thrombosis and pulmonary embolism is increased.