**Core Concept**
The superficial inguinal lymph nodes are primarily involved in the drainage of the lower limb, external genitalia (excluding the penile urethra), and the skin of the lower abdominal wall. Infection or inflammation in these areas can lead to enlargement of the superficial inguinal lymph nodes.

**Why the Correct Answer is Right**
The penile urethra primarily drains into the deep inguinal lymph nodes, which are located deeper in the inguinal canal. In contrast, the superficial inguinal lymph nodes receive lymphatic drainage from the skin of the penis, scrotum, and perineum, but not from the penile urethra itself. This is because the lymphatic vessels of the penile urethra follow the urethra into the deep inguinal nodes.

**Why Each Wrong Option is Incorrect**
**Option A:** The isthmus of the uterine tube, being part of the female reproductive system, does drain into the superficial inguinal lymph nodes. Infection or inflammation in this area can indeed cause enlargement of the superficial inguinal lymph nodes.

**Option B:** The inferior part of the anal canal does drain into the superficial inguinal lymph nodes. As a result, infection or inflammation in this area can lead to enlargement of the superficial inguinal lymph nodes.

**Option C:** The big toe, being part of the lower limb, does drain into the superficial inguinal lymph nodes. Infection or inflammation in this area can indeed cause enlargement of the superficial inguinal lymph nodes.

**Clinical Pearl / High-Yield Fact**
It's essential to remember the lymphatic drainage patterns of different body regions to accurately localize lymph node enlargement. The superficial inguinal nodes receive drainage from the lower limb, external genitalia (excluding the penile urethra), and the skin of the lower abdominal wall.

**✓ Correct Answer: D. Penile urethra**