Ketamine-induced urinary tract infection or sepsis: The recent history of subcutaneous injection of ketamine, which can be contaminated or lead to poor hygiene practices, increases the risk of infection. The symptoms of dark urine, fever, and chills suggest a possible urinary tract infection or sepsis.

Lymphogranuloma venereum (LGV): Given the patient's history of subcutaneous injection, which may imply high-risk sexual behavior, and the presence of left inguinal lymphadenopathy, LGV is a possible diagnosis. LGV is a sexually transmitted infection caused by certain types of Chlamydia trachomatis.
Tick-borne illnesses (e.g., Rocky Mountain spotted fever, ehrlichiosis): The patient lives in a tick-infested area, making tick-borne illnesses a consideration, especially with symptoms like fever and chills. However, the presence of lymphadenopathy and dark urine might be less typical for these conditions.
Cellulitis or abscess: The recent injection and the presence of lymphadenopathy could suggest a localized infection such as cellulitis or an abscess, especially if the injection site became infected.

Septic arthritis or osteomyelitis: Although less likely, if the patient has a history of intravenous drug use or the injection was not sterile, there's a risk of septic arthritis or osteomyelitis, which are medical emergencies.
Endocarditis: In the context of intravenous drug use, endocarditis is a potentially life-threatening condition that must be considered, even though the symptoms provided do not directly point towards it.
HIV or other blood-borne pathogen infection: Given the potential for shared needles or high-risk behaviors, testing for HIV and other blood-borne pathogens is crucial, as these infections can have significant implications for the patient's health and require prompt treatment.

Cat-scratch disease: Although less common, if the patient has been exposed to cats, Bartonella henselae infection (cat-scratch disease) could cause lymphadenopathy.
Toxoplasmosis: In immunocompromised individuals or those with specific exposures, toxoplasmosis could be considered, though it's less likely given the acute presentation and the specific symptoms described.
Lymphoma: While very rare and not directly suggested by the acute symptoms, in cases where lymphadenopathy does not resolve or is part of a broader systemic illness, lymphoma could be considered after other more likely causes are ruled out.