**Core Concept**
Winterbottom's sign is a clinical manifestation associated with African trypanosomiasis, also known as sleeping sickness. This sign is a significant indicator of the disease's presence and progression. It is characterized by the enlargement of lymph nodes in the posterior cervical region.

**Why the Correct Answer is Right**
Winterbottom's sign occurs due to the body's immune response to the trypanosome parasite, which causes the lymph nodes to become enlarged. This is a result of the parasite's invasion of the lymphatic system and the subsequent activation of the immune system. The posterior cervical lymphadenopathy is a distinctive feature of African trypanosomiasis and is often used as a diagnostic criterion. The lymph nodes may become tender and swollen, and in some cases, they may even rupture.

**Why Each Wrong Option is Incorrect**
**Option A:** Unilateral conjunctivitis is not a characteristic feature of sleeping sickness. While conjunctivitis can occur in some cases, it is not a specific sign of the disease.
**Option C:** Narcolepsy is a separate neurological disorder that is not related to African trypanosomiasis. It is characterized by excessive daytime sleepiness and is not associated with the enlargement of lymph nodes.
**Option D:** Transient erythema is a general term that refers to redness of the skin, which is not specific to sleeping sickness. While erythema may occur in some cases, it is not a distinctive feature of the disease.

**Clinical Pearl / High-Yield Fact**
Winterbottom's sign is a classic example of a clinical sign that is pathognomonic for a specific disease. It highlights the importance of careful physical examination in the diagnosis of infectious diseases. Remember that posterior cervical lymphadenopathy is a key feature of African trypanosomiasis and should prompt further investigation.

**✓ Correct Answer: B. Posterior cervical lymphadenopathy**