Which statement best describes the reason the first-voided morning urine is preferred for routine urinalysis?

• A. It is the most contaminated with microorganisms.
• B. It is the most concentrated, aiding detection of abnormalities.
• C. It contains the least protein.
• D. It has the most dilute chemical constituents.

Answer: (B)

The key idea is that the first-voided morning urine is more concentrated than urine collected later in the day. Overnight, urine remains in the bladder without being diluted by fluids, so it has higher solute concentration and a higher specific gravity. That increased concentration makes substances and formed elements easier to detect during routine urinalysis, improving the ability to identify abnormalities such as protein, glucose, ketones, blood, or crystals. It also reduces day-to-day variation from varying fluid intake and activity. It's not about contamination, and the morning sample isn’t inherently the one with the least protein or the most dilute chemical constituents—those would be less favorable for detecting abnormalities. For culture-related contamination issues, a clean-catch midstream specimen is typically used, but the morning sample’s advantage for routine UA lies in its concentrated nature.