## **Core Concept**
The **Prostate-Specific Antigen (PSA)** is a protein produced by the prostate gland. Elevated levels of PSA in the blood can indicate prostate cancer, prostatitis, or benign prostatic hyperplasia (BPH). The normal level of PSA varies slightly depending on the laboratory but generally falls within a specific range.

## **Why the Correct Answer is Right**
The correct answer, **< 4 ng/mL**, is considered the standard reference range for a normal PSA level in males. This value is based on the 95th percentile of the distribution of PSA levels in healthy men. Levels of PSA are influenced by age, and some studies suggest age-specific reference ranges (e.g., < 2.5 ng/mL for men under 60). However, 10 ng/mL - This is considered elevated and warrants further investigation, possibly including a biopsy.
- **Option B:** 4-10 ng/mL - This range is considered "gray zone" and may require further evaluation, such as a PSA velocity test or a biopsy.
- **Option D:** > 20 ng/mL - This is significantly elevated and strongly suggests prostate cancer or another serious condition.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **PSA test is not specific for cancer**; levels can be elevated in benign conditions. Also, some men with prostate cancer may have PSA levels within the "normal" range. Therefore, the decision to perform a biopsy or further testing depends on various factors, including the patient's age, overall health, and the results of a digital rectal examination (DRE).

## **Correct Answer:** C. < 4 ng/mL.