Final answer:
The diagnosis suggested by the symptoms described is Klinefelter syndrome, which involves an extra X chromosome and is associated with gynecomastia, hypogonadism, and an increased risk for gonadal malignancy.
Step-by-step explanation:
The description of an 18-year-old tall, lanky boy with mild mental reta*rdation (MR), gynecomastia, and hypogonadism, who also has an increased risk for gonadal malignancy, points to a likely diagnosis of Klinefelter syndrome. Klinefelter syndrome is characterized by the presence of an extra X chromosome in males (47,XXY) and can lead to various developmental and hormonal issues.
Normal male secondary sex characteristics like increased muscular development, growth of facial and body hair, and voice deepening may not fully develop in individuals with this condition. Additionally, the risk of testosterone deficiency and other effects on the male reproductive system may also be increased, similar to symptoms of andropause, but typically appearing much earlier in life.