A 55-year-old woman presents to her primary care physician with diarrhea. She states that it has persisted for the past several weeks and has not been improving. She also endorses episodes of feeling particularly flushed in the face. Her temperature is 99°F (37.2°C), blood pressure is 125/63 mmHg, pulse is 100/min, respirations are 15/min, and oxygen saturation is 97% on room air. Physical exam is notable for wheezing on pulmonary exam. The patient is discharged with medications for her symptoms. She returns 2 weeks later with symptoms of diarrhea, dry skin, a non-specific rash, and a notable decline in her memory. Which of the following is the most likely cause of this patient’s most recent presentation?

Increased vasoactive intestinal peptide levels

A 45-year-old man presents with lethargy, muscle aches, and dry skin. He is underweight and has very particular eating habits. Physical examination reveals swollen bleeding gums, cracked lips, petechiae, perifollicular hemorrhage, and corkscrew hairs. Laboratory tests reveal a nutritional deficiency. Which of the following is the key function of the most likely deficient nutrient?

Hydroxylation of lysine and proline residues in collagen synthesis

Component of the visual pigment rhodopsin

Gamma-carboxylation of glutamate residues in clotting factors

Cofactor in carboxylase reactions

A 17-year-old high school student presents to your office for recent mood and skin changes. The patient is a high school senior who is competing on the wrestling team and recently has lost weight to drop two weight classes over the past several months. He states he has dry, cracking, and irritated skin, as well as a sensation of tingling in his hands and feet. The patient also states that he has not been feeling himself lately. He finds himself more irritable and no longer enjoys many of the activities he once enjoyed. He finds that he often feels fatigued and has trouble concentrating. The patient does not have a significant past medical history and is not on any current medications. The patient admits to drinking alcohol and smoking marijuana on special occasions. He states that he uses supplements that his other team members use. Physical exam is significant for acne, dry, cracked skin around the patient's mouth in particular, and decreased sensation in his lower extremities. Laboratory values are as follows: Serum: Na+: 137 mEq/L Cl-: 101 mEq/L K+: 4.1 mEq/L HCO3-: 24 mEq/L BUN: 15 mg/dL Glucose: 79 mg/dL Creatinine: 0.9 mg/dL Ca2+: 9.2 mg/dL Mg2+: 1.5 mEq/L Homocysteine: 11.2 µmol/L (normal: 4.6 to 8.1 µmol/L) AST: 11 U/L ALT: 11 U/L Alkaline phosphatase: 27 U/L Albumin: 4.5 g/dL Total protein: 6.9 g/dL Total bilirubin: 0.5 mg/dL Direct bilirubin: 0.3 mg/dL Which of the following is the most likely diagnosis?

Water soluble vitamin deficiency

Depression secondary to dietary changes

A previously healthy 18-year-old woman comes to the physician because of a 2-day history of swelling and itchiness of her mouth and lips. It decreases when she eats cold foods such as frozen fruit. Four days ago, she underwent orthodontic wire-placement on her upper and lower teeth. Since then, she has been taking ibuprofen twice daily for the pain. For the past 6 months, she has been on a strict vegan diet. She is sexually active with one partner and uses condoms consistently. She had chickenpox that resolved spontaneously when she was 6 years old. Her vitals are within normal limits. Examination shows diffuse erythema and edema of the buccal mucosa with multiple serous vesicles and shallow ulcers. Stroking the skin with pressure does not cause blistering of the skin. The remainder of the examination shows no abnormalities. Which of the following is the most likely cause of these symptoms?

Reactivation of varicella zoster virus

Deficiency syndromes of water-soluble vitamins for USMLE Step 2 CK: High-Yield Biochemistry Lessons

Thiamine (B1) & Riboflavin (B2) - The Energy Duo

Thiamine (B1): Cofactor for dehydrogenase enzymes (Pyruvate DH, α-ketoglutarate DH, Transketolase).
- Deficiency: Impaired glucose breakdown → ATP depletion.
- Wernicke-Korsakoff Syndrome: Classic triad of Confusion, Ophthalmoplegia, Ataxia (📌 COA). Common in alcoholism. Progresses to irreversible memory loss & confabulation (Korsakoff psychosis).
- Beriberi:
  - Dry: Symmetrical peripheral neuropathy.
  - Wet: High-output, dilated cardiomyopathy; edema.

⭐ Giving glucose to a B1-deficient patient without thiamine can precipitate acute Wernicke encephalopathy.

Riboflavin (B2): Component of FAD & FMN for redox reactions.
- Deficiency: Cheilosis (fissures at mouth corners), Corneal vascularization, Magenta-colored tongue (📌 The 2 C's of B2).

Niacin (B3) & Pyridoxine (B6) - Pellagra Players

Niacin (B3) Deficiency (Pellagra)
- 📌 3 D's: Dermatitis (photosensitive, Casal necklace), Diarrhea, Dementia. Fourth D is Death.
- Causes: Hartnup disease, carcinoid syndrome, isoniazid.
- Synthesized from Tryptophan; requires B2 and B6.
Pyridoxine (B6) Deficiency
- Causes: Isoniazid, oral contraceptives.
- Symptoms: Sideroblastic anemia (impaired heme synthesis), peripheral neuropathy, cheilosis, glossitis, convulsions.

⭐ Isoniazid can cause a functional deficiency of both B3 and B6. It inhibits pyridoxal kinase (activating B6) and can lead to pellagra-like symptoms by depleting the B6 needed for niacin synthesis.

Pellagra: Casal necklace dermatitis

Folate (B9) & Cobalamin (B12) - Megaloblastic Madness

Shared Pathology: Impaired DNA synthesis leads to megaloblastic anemia (MCV > 100 fL), pancytopenia, hypersegmented neutrophils (>5 lobes), and glossitis.
Lab Markers: Both deficiencies cause ↑ homocysteine. Only B12 deficiency causes ↑ methylmalonic acid (MMA), as B12 is a cofactor for methylmalonyl-CoA mutase.

⭐ Folate supplementation can mask the hematologic signs of B12 deficiency, permitting irreversible neurological damage (subacute combined degeneration) to progress.

📌 Folate from foliage; B12 from animal biproducts.

Vitamin C & Friends - Scurvy Crew

Scurvy: perifollicular hemorrhage and corkscrew hairs

Function: Ascorbic acid is a key antioxidant and essential cofactor for:
- Collagen Synthesis: Prolyl & lysyl hydroxylases. Deficiency → fragile vessels & poor wound healing.
- Dopamine Synthesis: Dopamine β-hydroxylase (converts dopamine to NE).
- Iron Absorption: Reduces dietary ferric ($Fe^{3+}$) to absorbable ferrous ($Fe^{2+}$) iron in the gut.
Deficiency (Scurvy): Presents after months of deprivation.
- Mucocutaneous: Swollen, bleeding gums; perifollicular hemorrhage; petechiae; coiled "corkscrew" hairs.
- MSK: Hemarthrosis, subperiosteal hemorrhage.
- Systemic: Anemia, malaise, impaired wound healing.

⭐ High-Yield: Vitamin C deficiency can cause microcytic anemia due to its role in iron absorption and macrocytic anemia if folate intake is also poor.

High‑Yield Points - ⚡ Biggest Takeaways

Thiamine (B1) deficiency causes Wernicke-Korsakoff syndrome (confusion, ataxia, ophthalmoplegia) and beriberi.

Niacin (B3) deficiency leads to pellagra: the 3 Ds of dermatitis, diarrhea, and dementia.

Folate (B9) deficiency results in megaloblastic anemia and neural tube defects without neurologic symptoms.

Cobalamin (B12) deficiency causes megaloblastic anemia with irreversible neurological deficits.

Vitamin C deficiency (scurvy) presents with impaired wound healing, bleeding gums, and perifollicular hemorrhage.

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