**Core Concept**
Anorexia Nervosa (AN) is a psychiatric disorder characterized by a distorted body image and an intense fear of gaining weight, leading to extreme restriction of food intake and significant weight loss. The condition often presents with physical manifestations such as callous formation on the knuckles (Russell's sign), bradycardia, and hypotension.

**Why the Correct Answer is Right**
The patient's history of restrictive eating, excessive vomiting after meals, and extremely low BMI (15 kg/m2) are indicative of Anorexia Nervosa. The presence of callous formation on the knuckles (Russell's sign) further supports this diagnosis, as it is a classic physical finding associated with AN. This condition is often linked to the hypothalamic-pituitary-adrenal (HPA) axis dysfunction, leading to hormonal imbalances, particularly a decrease in leptin levels, which contribute to the development of AN.

**Why Each Wrong Option is Incorrect**
**Option A:** Bulimia Nervosa (BN) is characterized by recurrent episodes of binge eating followed by compensatory behaviors like self-induced vomiting or laxative abuse. However, the patient's extremely small meal size and low BMI are not typical of BN.
**Option B:** Avoidant/Restrictive Food Intake Disorder (ARFID) is characterized by a lack of interest in eating or avoidance of certain foods due to sensory issues or fear of negative consequences. However, ARFID does not involve the same level of psychological distress or fear of gaining weight as seen in AN.
**Option C:** Other Specified Feeding or Eating Disorder (OSFED) is a category for eating disorders that do not meet the full criteria for AN, BN, or ARFID. However, the patient's presentation is more consistent with AN than OSFED.

**Clinical Pearl / High-Yield Fact**
Russell's sign, also known as callous formation on the knuckles, is a classic physical finding associated with Anorexia Nervosa. It is caused by the patient's habit of inducing vomiting by forcing their fingers down their throat, leading to callous formation on the knuckles.

**Correct Answer:** C.