The basic structural and functional unit of the nervous system is: MCQ with Answer and Explanation

Explanation:
The 'lub' sound occurs at the beginning of ventricular systole when the atrioventricular valves (mitral and tricuspid) close to prevent backflow of blood into the atria. The 'dub' sound occurs at the beginning of diastole due to closure of the semilunar valves (aortic and pulmonary). Valve closure is passive, resulting from pressure gradients.

Explanation:
Klinefelter syndrome is a sex chromosomal aneuploidy in males characterized by the karyotype 47, XXY. Individuals are phenotypically male but often exhibit feminine characteristics such as gynecomastia (development of breasts), sparse body hair, and are typically sterile due to underdeveloped testes. Down syndrome is trisomy 21 (47, +21), Turner syndrome is 45, X0, and Patau syndrome is trisomy 13.

Explanation:
The question describes sieve elements, which are the conducting cells of the phloem, not xylem. Sieve tube elements are living but lack a nucleus at maturity and have perforated end walls called sieve plates. Xylem vessels and tracheids are dead, hollow, and lack perforated end walls (vessels have perforation plates, but they are not called sieve plates). Xylem parenchyma is living.