**Core Concept**
Vitamin C (ascorbic acid) is an essential nutrient for collagen synthesis and iron absorption. Its deficiency can lead to impaired wound healing, easy bruising, and bleeding gums due to impaired collagen cross-linking.

**Why the Correct Answer is Right**
The patient's symptoms of easy bruising, bleeding gums, and perifollicular hemorrhages are classic signs of vitamin C deficiency. Vitamin C is necessary for the hydroxylation of proline and lysine residues in collagen, which is essential for collagen cross-linking and stabilization. The patient's normal coagulation profile and liver function suggest that the issue is not related to a bleeding disorder or liver dysfunction. Instead, the symptoms point towards a dietary deficiency of vitamin C.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because thiamine deficiency primarily affects the nervous system and heart, leading to conditions like beriberi and Wernicke-Korsakoff syndrome. The patient's symptoms do not align with thiamine deficiency.

**Option B:** This option is incorrect because vitamin K is essential for blood clotting, and its deficiency would lead to coagulopathy, which is not the case in this patient.

**Option C:** This option is incorrect because vitamin B12 deficiency primarily affects the nervous system and hematopoiesis, leading to conditions like megaloblastic anemia and peripheral neuropathy. The patient's symptoms do not align with vitamin B12 deficiency.

**Clinical Pearl / High-Yield Fact**
Patients with vitamin C deficiency may also exhibit symptoms like fatigue, malaise, and poor wound healing. Additionally, vitamin C deficiency can increase the risk of infections and impaired iron absorption.

**Correct Answer:** C. Vitamin C deficiency