**Core Concept**
Retention of urine in the reproductive age group can be caused by various factors, including anatomical abnormalities, pelvic masses, and urinary tract infections. The causes can be broadly classified into obstructive and non-obstructive.

**Why the Correct Answer is Right**
Retention of urine can occur due to obstruction of the urinary tract. A retroverted gravid uterus can cause compression of the urethra or bladder, leading to urinary retention. This occurs because the uterus, being retroverted, can put pressure on the bladder and urethra, obstructing the flow of urine.

**Why Each Wrong Option is Incorrect**
**Option A:** Cervical fibroid - While cervical fibroids can cause pelvic pressure and discomfort, they are less likely to cause urinary retention directly. Cervical fibroids may cause issues with urination, but they are not a common cause of retention of urine.

**Option B:** Unilateral hydronephrosis - Hydronephrosis refers to the swelling of a kidney due to a blockage in the urinary tract. While it can cause urinary retention, it is not the most direct cause in the reproductive age group. Additionally, unilateral hydronephrosis would not typically cause complete urinary retention.

**Option D:** Severe UTI - A severe urinary tract infection can cause urinary retention by causing inflammation and spasms in the bladder and urethra. However, it is not the most common cause of retention of urine in the reproductive age group.

**Option E:** Posterior urethral valve - Posterior urethral valves are a congenital anomaly that can cause urinary retention in males. However, this condition is more common in neonates and young children, not in the reproductive age group.

**Clinical Pearl / High-Yield Fact**
A retroverted gravid uterus is a common cause of urinary retention in the reproductive age group, especially during the second and third trimesters of pregnancy. It is essential to diagnose and manage this condition promptly to prevent complications.

**Correct Answer:** B. Retroverted gravid uterus.