Okay, let's tackle this question. The patient is a 17-year-old girl with acne who's been on a medication for two years and now has blue-black nail pigmentation. The key here is to figure out which drug causes such a side effect.

First, I need to recall drugs that are used for acne. Common ones include topical retinoids like tretinoin, antibiotics like tetracyclines, and isotretinoin. But blue-black pigmentation isn't typical for those. Wait, there's another class: antimalarials like hydroxychloroquine can cause nail pigmentation, but they're not used for acne. Then there's minocycline, a tetracycline antibiotic. Oh right! Minocycline is used for acne and is known for causing blue-gray skin pigmentation, including the nails.

So the correct answer is likely minocycline. Let's check the options. The question doesn't list them, but based on the scenario, minocycline is the drug. The blue-black pigmentation is a known side effect due to it being a photosensitive drug that deposits in melanin-containing cells.

Other options might include other tetracyclines, but doxycycline isn't as likely. Doxycycline can cause discoloration too, but more commonly in teeth, not nails. Then there's tetracycline itself, but minocycline is more associated with this. Also, isotretinoin can cause other side effects like dry skin but not pigmentation.

The clinical pearl here is that minocycline's pigmentation is a classic side effect, so students should remember that when they see blue-black nail changes in a patient on long-term acne medication.

**Core Concept**
Blue-black nail pigmentation is a rare but recognized side effect of long-term minocycline use. This occurs due to the drug's deposition in melanin-containing cells and its photosensitive properties, which catalyze oxidative damage to melanin.

**Why the Correct Answer is Right**
Minocycline, a second-generation tetracycline antibiotic used for acne, can cause dose-dependent, photo-enhanced melanin oxidation in melanocytes. This leads to blue-gray to black pigmentation of the skin, nails, and mucous membranes. The pigmentation is typically irreversible even after discontinuation.

**Why Each Wrong Option is Incorrect**
**Option A:** Doxycycline causes brownish discoloration of teeth in children but not blue-black nail pigmentation.
**Option B:** Isotretinoin (a retinoid) may cause dryness, cheilosis, or elevated liver enzymes but not nail pigmentation.
**Option C:** Topical tretinoin (a topical retinoid) is used for mild acne and does not penetrate to cause systemic pigmentation.

**Clinical Pearl / High-Yield Fact**
Minocycline-induced pigmentation is a classic exam trap—remember "blue nails and gray skin" with long-term use. Discontinuation is often necessary if the side effect is severe.

**Correct Answer:** C. Minocycline

✓ Correct Answer: B. Minocycline