**Core Concept**
Pitting of nails, also known as nail pitting, is a clinical sign characterized by small depressions or pits on the surface of the fingernails or toenails. This condition can be associated with various systemic and dermatological disorders.

**Why the Correct Answer is Right**
Nail pitting is a common feature in psoriasis, an autoimmune skin condition characterized by thickened, scaly skin lesions. The exact mechanism of nail pitting in psoriasis is not fully understood, but it is believed to be related to the abnormal keratinization of nail cells and the disruption of nail plate growth. Lichen planus, another autoimmune condition, can also cause nail pitting due to the inflammation and scarring of the nail matrix. Arsenic poisoning can cause a range of skin and nail abnormalities, including nail pitting, due to the toxic effects of arsenic on the nail matrix and surrounding tissues.

**Why Each Wrong Option is Incorrect**
**Option B:** Pemphigus is a group of autoimmune skin disorders characterized by blistering of the skin and mucous membranes, but it is not typically associated with nail pitting.
**Option C:** Arsenic poisoning is a correct option, but the question asks for a combination of conditions that can cause nail pitting. While arsenic poisoning can cause nail pitting, it is not the only condition listed that can cause this symptom.
**Option D:** This option includes pemphigus, which is not typically associated with nail pitting.

**Clinical Pearl / High-Yield Fact**
Nail pitting can be a subtle but important clinical sign of underlying systemic or dermatological disorders. It is essential for clinicians to carefully examine the nails and consider the possibility of associated conditions, particularly in patients with psoriasis or lichen planus.

**✓ Correct Answer: A. ab. Psoriasis and Lichen planus can cause nail pitting.**