**Core Concept**
The question pertains to the treatment of both partners in certain infectious diseases, specifically highlighting the importance of dual therapy in preventing transmission and recurrence. This approach is crucial in managing sexually transmitted infections (STIs) and other communicable diseases that can be transmitted through sexual contact.

**Why the Correct Answer is Right**
In many STIs, such as syphilis and chlamydia, treating both partners is essential to prevent reinfection and reduce the risk of transmission to others. This is because these infections can be asymptomatic in one partner, making them unaware of their infection status. By treating both partners, healthcare providers can help ensure that the infection is fully cleared and reduce the risk of transmission.

**Why Each Wrong Option is Incorrect**
**Option A:**
Treating both partners is not always mandatory, but it is highly recommended in certain situations, such as when one partner has a history of reinfection or when the infection is severe. However, the specific scenarios where dual therapy is mandatory may vary depending on the disease and local guidelines.

**Option B:**
In some cases, treating only one partner may be sufficient, especially if the other partner is asymptomatic or has a proven negative test result. However, this approach should be taken with caution and under the guidance of a healthcare provider.

**Option C:**
Treating both partners is mandatory in all cases of gonorrhea, which is a common STI that can cause severe complications if left untreated. Dual therapy is recommended to ensure that the infection is fully cleared and to reduce the risk of antibiotic resistance.

**Option D:**
Treating both partners is mandatory in all cases of HIV, which is a chronic and potentially life-threatening condition. Dual therapy is essential to prevent transmission and ensure that both partners receive the necessary treatment and support.

**Clinical Pearl / High-Yield Fact**
When treating STIs, it's essential to consider the potential for asymptomatic infection and the importance of dual therapy in preventing transmission and recurrence. This approach can help reduce the risk of complications and ensure that both partners receive the necessary treatment and support.

**Correct Answer: C. Treating both partners is mandatory in all cases of gonorrhea.**