Testicular artery ligation: This is a known complication of inguinal hernia repair, where the testicular artery is inadvertently ligated during the procedure, leading to ischemia and atrophy of the testis. The timing of one month post-surgery fits with this diagnosis, as it allows enough time for the testicular atrophy to become apparent.

Tight external ring of the inguinal canal: If the external ring was not adequately relaxed or was too tight during the hernia repair, it could lead to compression of the testicular vessels, resulting in testicular atrophy.
Thrombosis of the pampiniform plexus: Although less common, thrombosis of the pampiniform plexus (a network of veins that drain the testis) could occur as a complication of the surgery, leading to testicular ischemia and atrophy.

Mesh prolapsed into the scrotum: While less likely, if mesh used in the hernia repair were to prolapse into the scrotum, it could potentially compress the testicular vessels or cause other complications leading to testicular atrophy. This diagnosis is critical to consider because it might require urgent intervention to prevent further complications.

Idiopathic testicular atrophy: In some cases, testicular atrophy can occur without a clear cause, possibly due to factors unrelated to the surgery. However, given the recent surgical history, this would be a less likely explanation.
Other vascular complications: Other rare vascular complications, such as vasculitis or spontaneous thrombosis not related to the surgery, could theoretically lead to testicular atrophy, but these would be uncommon and typically associated with other systemic symptoms or risk factors.