**Core Concept**
Postpartum uterine infection, also known as puerperal infection, is a common complication following childbirth. It can manifest as endometritis, parametritis, salpingitis, or vaginitis, depending on the site of infection. Understanding the typical presentation and clinical features of each condition is crucial for accurate diagnosis and management.

**Why the Correct Answer is Right**
Endometritis is the most common and severe form of puerperal infection, typically occurring 4-10 days postpartum. It is characterized by inflammation of the endometrium, often as a result of bacterial contamination during delivery. The patient's symptoms, including fever, uterine tenderness, and a soft uterus, are consistent with endometritis. In this condition, the endometrium becomes edematous and hemorrhagic, leading to a foul odor and bleeding.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vaginitis refers to inflammation of the vagina, often caused by infection or irritation. While vaginitis can occur postpartum, the patient's symptoms and examination findings are more suggestive of endometritis.
* **Option C:** Parametritis is inflammation of the parametrium, a layer of connective tissue surrounding the uterus. Although parametritis can occur in association with endometritis, the primary site of infection in this case appears to be the endometrium.
* **Option D:** Salpingitis is inflammation of the fallopian tubes, often caused by pelvic inflammatory disease (PID). While salpingitis can occur postpartum, the patient's symptoms and examination findings are more consistent with endometritis.

**Clinical Pearl / High-Yield Fact**
In the postpartum period, a soft, tender uterus with fever and uterine tenderness is highly suggestive of endometritis. Prompt recognition and treatment of endometritis are essential to prevent complications and ensure a safe recovery for the mother.

**✓ Correct Answer: B. Endometritis**