**Core Concept**
Isoniazid, a first-line antitubercular medication, is known to interfere with the body's tryptophan metabolism, leading to a deficiency in niacin (vitamin B3). This deficiency can cause a range of symptoms, including dermatitis, diarrhea, and cognitive impairment.

**Why the Correct Answer is Right**
Isoniazid inhibits the enzyme tryptophan pyrrolase, which is responsible for converting tryptophan into niacin in the liver. As a result, the patient's body is unable to produce sufficient amounts of niacin, leading to a deficiency. The symptoms described, including rashes on exposed areas of the body, disoriented memory, and diarrhea, are all classic manifestations of niacin deficiency. This condition is also known as pellagra.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid neuropathy is a known side effect of isoniazid therapy, but it typically presents with numbness, tingling, or weakness in the hands and feet, rather than the symptoms described.

**Option B:** Tuberculosis skin lesions can occur in patients with active TB, but they are typically localized to the site of infection and do not cause systemic symptoms like diarrhea or cognitive impairment.

**Option D:** While other medications can cause a range of side effects, the specific combination of symptoms described in the question is highly suggestive of niacin deficiency.

**Clinical Pearl / High-Yield Fact**
Pellagra, caused by niacin deficiency, is characterized by the "three Ds": dermatitis, diarrhea, and dementia. This condition is often seen in patients who are malnourished or have a diet that is deficient in tryptophan.

**✓ Correct Answer: C. Niacin deficiency**