Folliculitis: This is the most likely diagnosis given the itchy rash with bumps and small pustules on a shaved neck. Folliculitis is an inflammation of the hair follicles, often caused by bacterial infection (most commonly Staphylococcus aureus) and can be exacerbated by shaving, which can cause micro-trauma to the skin and introduce bacteria into the follicles.

Razor Burn (Pseudofolliculitis Barbae): This condition is common in individuals who shave, especially if they have curly hair. It occurs when hairs curve back into the skin, causing inflammation and leading to bumps and pustules.
Contact Dermatitis: This could be due to an allergic reaction or irritation from shaving cream, aftershave, or other products used on the skin. It presents with redness, itching, and small bumps.
Ingrown Hairs: Similar to razor burn, ingrown hairs can cause bumps and pustules, especially in areas where the hair is curly and the skin is sensitive.

Methicillin-Resistant Staphylococcus aureus (MRSA) Infection: While less common, MRSA can cause folliculitis or skin infections that resemble the symptoms described. It's crucial to consider this diagnosis, especially if the infection does not respond to typical treatments, as MRSA requires specific antibiotic therapy.
Herpes Simplex Virus (HSV) Infection: Although less likely, HSV can cause a rash with vesicles or pustules. Given the potential for serious complications, especially if the infection spreads to the eyes, it's essential to consider HSV in the differential diagnosis.

Keratosis Pilaris: A condition characterized by small, rough bumps on the skin, often on the arms, legs, buttocks, and cheeks, but can occasionally appear on the neck. It's usually associated with dry skin and can be itchy.
Eczema (Atopic Dermatitis): While more common in children, atopic dermatitis can occur in adults and may present with itchy, inflamed skin that can appear anywhere on the body, including the neck.
Tinea Barbae (Fungal Infection of the Beard Area): A fungal infection that can cause itchy, circular patches with redness and swelling in the beard area, including the neck. It's less common than bacterial folliculitis but should be considered, especially if the patient has been in contact with someone with a similar infection or has recently traveled.