**Core Concept**
The underlying principle being tested involves the anatomical and physiological changes during pregnancy, specifically in the 2nd trimester. **Urethral compression** and **bladder neck obstruction** can occur due to the expanding **uterus**.

**Why the Correct Answer is Right**
Although the exact options are not provided, the most probable cause of urinary retention in a pregnant woman, especially in the 2nd trimester, often relates to mechanical factors. The expanding **gravid uterus** can compress the **urethra**, leading to obstruction and subsequent retention. This is more common in women with a **retroverted uterus** or those carrying a **foetus** in a position that exacerbates urethral compression.

**Why Each Wrong Option is Incorrect**
**Option A:** Would be incorrect if it suggested a neurological cause, as these are less common in the 2nd trimester without other symptoms.
**Option B:** Might be wrong if it implicated a direct **bladder** issue, as these are not typically caused by pregnancy itself in the 2nd trimester.
**Option C:** Could be incorrect if it proposed a **hormonal** imbalance, as while hormones play a role in pregnancy, they're not the primary cause of urinary retention in this context.
**Option D:** Would be incorrect if it suggested an **infectious** cause, as infections would present with additional symptoms like dysuria or fever.

**Clinical Pearl / High-Yield Fact**
Urinary retention in pregnancy is a significant issue that can lead to **renal** complications if not addressed. It's crucial to identify and manage the underlying cause, often through **catheterization** or adjusting the pregnant woman's position to alleviate **urethral compression**.

**Correct Answer:** D. Mechanical obstruction due to the gravid uterus.