**Core Concept**
Hydroquinone is a topical medication used for the treatment of various hyperpigmentation disorders, including melasma, freckles, and solar lentigines. It works by inhibiting the enzyme tyrosinase, which plays a crucial role in the production of melanin. Hydroquinone is classified as a monobenzyl ether of dihydroxybenzene.

**Why the Correct Answer is Right**
Hydroquinone is generally considered safe and effective for treating melasma and chloasma of pregnancy when used under the guidance of a dermatologist. However, option D states that it should not be used for these conditions. This is incorrect because hydroquinone is actually considered a first-line treatment for melasma and chloasma of pregnancy. The American Academy of Dermatology recommends hydroquinone as a treatment option for these conditions due to its efficacy in reducing melanin production.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect because hydroquinone is actually considered a strong hypopigmenting agent, not weak. It is effective in reducing melanin production and is often used to treat various hyperpigmentation disorders.

**Option B:** This statement is correct, as hydroquinone does inhibit tyrosinase, an enzyme involved in the production of melanin. This inhibition leads to a decrease in melanin production, making hydroquinone an effective treatment for hyperpigmentation disorders.

**Option C:** This statement is correct, as hydroquinone can cause incomplete responses and pigmentation may recur. This is because hydroquinone only inhibits melanin production, it does not eliminate melanocytes entirely. Therefore, pigmentation may recur once treatment is stopped.

**Clinical Pearl / High-Yield Fact**
It is essential to note that hydroquinone should be used with caution and under the guidance of a dermatologist due to its potential side effects, such as skin irritation, contact dermatitis, and ochronosis (a condition characterized by bluish-gray skin discoloration).

**✓ Correct Answer: D. It should not be used for melasma or chloasma of pregnancy**