**Core Concept**
During pregnancy, the uterus expands and compresses the urinary bladder, leading to changes in bladder physiology and function. The increased abdominal pressure and stretching of the bladder detrusor muscle can cause incomplete bladder emptying and urinary retention.

**Why the Correct Answer is Right**
The likely cause of acute retention of urine in a 10-12 weeks pregnant lady is **urethral compression**. As the uterus expands, it can compress the urethra, making it difficult for the bladder to empty. This is due to the increased intra-abdominal pressure and the mechanical compression of the urethra by the expanding uterus. The detrusor muscle, which is responsible for bladder contraction, may also be affected by the increased pressure.

**Why Each Wrong Option is Incorrect**
**Option A:** The patient is not old enough to be considered postmenopausal, making atrophic urethritis (a common cause of urinary retention in postmenopausal women) an unlikely cause.
**Option B:** The patient is not experiencing symptoms of a urinary tract infection (UTI), which can cause urinary retention due to pain and discomfort.
**Option C:** The patient is not experiencing symptoms of a pelvic mass or tumor, which can cause urinary retention due to mechanical compression of the urethra or bladder.

**Clinical Pearl / High-Yield Fact**
In pregnant patients, it's essential to consider the mechanical effects of the expanding uterus on the urinary tract, particularly in the second and third trimesters when the uterus is larger.

**Correct Answer:** C. Urethral compression.