Down syndrome is a genetic disorder caused by the presence of an extra copy of chromosome 21. It is also known as trisomy 21, because affected individuals have three copies of chromosome 21 instead of the normal two.

The extra chromosome 21 in Down syndrome occurs due to a random error in cell division, which leads to the production of an abnormal gamete (egg or sperm) with an extra copy of the chromosome. When this gamete fuses with a normal gamete during fertilization, the resulting zygote has 47 chromosomes instead of the usual 46, and develops into a fetus with Down syndrome.

Down syndrome is characterized by a range of physical and intellectual symptoms, including developmental delays, intellectual disability, distinctive facial features, heart defects, and increased risk of certain medical conditions such as leukemia and Alzheimer's disease. However, the severity and expression of these symptoms can vary widely among affected individuals.

• A. Nanolitres (nL) → Extremely small unit (1 nL = 0.000000001 L), used for microscopic measurements. ❌
• B. Microlitres (µL) → Also very small (1 µL = 0.000001 L), used in lab settings. ❌
• C. Litres (L) → Too large, as typical urine output per hour is much less than 1 L. ❌
• D. Millilitres (mL) → Suitable, since normal urine output per hour is about 30-80 mL. ✅

• A. Adduction → Moving a limb toward the midline of the body. ❌
• B. Extension → Increasing the angle of a joint (e.g., straightening the arm or leg). ❌
• C. Flexion → Decreasing the angle of a joint (e.g., bending the elbow or knee). ❌
• D. Abduction → Moving a limb away from the midline (e.g., raising the arm or leg sideways). ✅

• Enzymatic reactions follow a Michaelis-Menten curve.
• As substrate concentration increases, the reaction rate initially increases rapidly.
• However, at high substrate concentrations, the enzyme becomes saturated, meaning all active sites are occupied, and adding more substrate does not increase the reaction rate further.
• This leads to a plateau in reaction rate, which is represented by line A.

Analysis of the Other Lines:

• B: Shows a linear decrease, which does not represent enzyme kinetics. ❌
• C: Depicts a constant reaction rate independent of substrate concentration, which is incorrect. ❌
• D: Shows a decreasing reaction rate, which does not align with enzyme behavior. ❌

The data collected by the researcher on the number of cars passing through a busy intersection at different times of the day for a month would be most useful to analyze traffic patterns during rush hour.

• A. Sarcoplasmic Reticulum → A specialized endoplasmic reticulum in muscle cells that stores and releases calcium for muscle contraction. ❌
• B. Muscle Fiber → Another name for a muscle cell, which is a long, cylindrical, multinucleated cell that makes up muscle tissue. ✅
• C. Sarcolemma → The cell membrane of a muscle fiber, which surrounds the muscle cell. ❌
• D. Myofilament → Protein structures (actin and myosin) within a muscle fiber that are responsible for contraction. ❌

The largest organ in the human body by surface area is the skin. It covers the enre external surface of the body and has an average surface area of about 20 square feet in adults.

Exocrine glandular is not one of the four primary ssue types found in the human body. The four primary ssue types are epithelial, nervous, connective, and muscle.

The diaphragm is responsible for regulating breathing rate and depth. It is a dome-shaped muscle located at the bottom of the chest cavity that contracts and relaxes to help move air in and out of the lungs.